Optical waveguide structures

ABSTRACT

The purely bound electromagnetic modes of propagation supported by waveguide structures comprised of a thin lossy metal film of finite width embedded in an infinite homogeneous dielectric have been characterized at optical wavelengths. One of the fundamental modes supported by the structure exhibits very interesting characteristics and is potentially quite useful. It evolves with decreasing film thickness and width towards the TEM wave supported by the background (an evolution similar to that exhibited by the s b  mode in symmetric metal film slab waveguides), its losses and phase constant tending asymptotically towards those of the TEM wave. Attenuation values can be well below those of the s b  mode supported by the corresponding metal film slab waveguide. Low mode power attenuation in the neighbourhood of 10 to 0.1 dB/cm is achievable at optical communications wavelengths, with even lower values being possible. Carefully selecting the film&#39;s thickness and width can make this mode the only long-ranging one supported. In addition, the mode can have a field distribution that renders it excitable using an end-fire approach. The existence of this mode renders the finite-width metal film waveguide attractive for applications requiring short propagation distances and 2-D field confinement in the transverse plane, enabling various devices to be constructed, such as couplers, splitters, modulators, interferometers, switches and periodic structures. Under certain conditions, an asymmetric structure can support a long-ranging mode having a field distribution that is suitable to excitation using an end-fire technique. Like asymmetric slab waveguides. The attenuation of the long-ranging mode near cutoff decreases very rapidly, much more so than the attenuation related to the long-ranging mode in a similar symmetric structure. The cutoff thickness of a long-ranging mode in an asymmetric finite-width structure is larger than the cutoff thickness of the s b  mode in a similar asymmetric slab waveguide. This implies that the long-ranging mode supported by an asymmetric finite-width structure is more sensitive to the asymmetry in the structure compared to the s b  mode supported by a similar slab waveguide. This result is interesting and potentially useful in that the propagation of such a mode can be affected by a smaller change in the dielectric constant of the substrate or superstrate compared with similar slab structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a Continuation-in-Part of U.S. patent application Ser.No. 09/742,422 which itself was a Continuation-in-Part of U.S. patentapplication Ser. No. 09/629,816 filed Jul. 31, 2000, now U.S. Pat. No.6,442,321 issued Aug. 27, 2002, and which claimed priority from U.S.Provisional patent application No. 60/171,606 filed Dec. 23, 1999,Canadian patent application No. 2,314,723 filed Jul. 31, 2000 andCanadian patent application No. 2,319,949 filed Sep. 20, 2000. Thecontents of these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] The invention relates to optical devices and is especiallyapplicable to waveguide structures and integrated optics.

[0004] 2. Background Art

[0005] This specification refers to several published articles. Forconvenience, the articles are cited in full in a numbered list at theend of the description and cited by number in the specification itself.The contents of these articles are incorporated herein by reference andthe reader is directed to them for reference.

[0006] In the context of this patent specification, the term “opticalradiation” embraces electromagnetic waves having wavelengths in theinfrared, visible and ultraviolet ranges.

[0007] The terms “finite” and “infinite” as used herein are used bypersons skilled in this art to distinguish between waveguides having“finite” widths in which the actual width is significant to theperformance of the waveguide and the physics governing its operation andso-called “infinite” waveguides where the width is so great that it hasno significant effect upon the performance and physics or operation.

[0008] At optical wavelengths, the electromagnetic properties of somemetals closely resemble those of an electron gas, or equivalently of acold plasma. Metals that resemble an almost ideal plasma are commonlytermed “noble metals” and include, among others, gold, silver andcopper. Numerous experiments as well as classical electron theory bothyield an equivalent negative dielectric constant for many metals whenexcited by an electromagnetic wave at or near optical wavelengths [1,2].In a recent experimental study, the dielectric function of silver hasbeen accurately measured over the visible optical spectrum and a veryclose correlation between the measured dielectric function and thatobtained via the electron gas model has been demonstrated [3].

[0009] It is well-known that the interface between semi-infinitematerials having positive and negative dielectric constants can guide TM(Transverse Magnetic) surface waves. In the case of a metal-dielectricinterface at optical wavelengths, these waves are termedplasmon-polariton modes and propagate as electromagnetic fields coupledto surface plasmons (surface plasma oscillations) comprised ofconduction electrons in the metal [4].

[0010] It is known to use a metal film of a certain thickness bounded bydielectrics above and below as an optical slab (planar, infinitely wide)waveguiding structure, with the core of the waveguide being the metalfilm. When the film is thin enough, the plasmon-polariton modes guidedby the interfaces become coupled due to field tunnelling through themetal, thus creating supermodes that exhibit dispersion with metalthickness. The modes supported by infinitely wide symmetric andasymmetric metal film structures are well-known, as these structureshave been studied by numerous researchers; some notable published worksinclude references [4] to [10].

[0011] In general, only two purely bound TM modes, each having threefield components, are guided by an infinitely wide metal film waveguide.In the plane perpendicular to the direction of wave propagation, theelectric field of the modes is comprised of a single component, normalto the interfaces and having either a symmetric or asymmetric spatialdistribution across the waveguide. Consequently, these modes are denoteds_(b) and a_(b) modes, respectively. The s_(b) mode can have a smallattenuation constant and is often termed a long-range surfaceplasmon-polariton. The fields related to the a_(b) mode penetratefurther into the metal than in the case of the s_(b) mode and can bemuch lossier by comparison. Interest in the modes supported by thinmetal films has recently intensified due to their useful application inoptical communications devices and components. Metal films are commonlyemployed in optical polarizing devices [11] while long-range surfaceplasmon-polaritons can be used for signal transmission [7]. In additionto purely bound modes, leaky modes are also known to be supported bythese structures.

[0012] Infinitely wide metal film structures, however, are of limitedpractical interest since they offer one-dimensional (1-D) fieldconfinement only, with confinement occurring along the vertical axisperpendicular to the direction of wave propagation, implying that modeswill spread out laterally as they propagate from a point source used asthe excitation. Metal films of finite width have recently been proposedin connection with polarizing devices [12], but merely as a cladding.

[0013] In addition to the lack of lateral confinement, plasmon-polaritonwaves guided by a metal-dielectric interface are in general quite lossy.Even long-range surface plasmons guided by a metal film can be lossy bycomparison with dielectric waveguides. Known devices exploit this highloss associated with surface plasmons for the construction ofplasmon-polariton based modulators and switches. Generally, knownplasmon-polariton based modulator and switch devices can be classifiedalong two distinct architectures. The first architecture is based on thephenomenon of attenuated total reflection (ATR) and the secondarchitecture is based on mode coupling between a dielectric waveguideand a nearby metal. Both architectures depend on the dissipation ofoptical power within an interacting metal structure.

[0014] ATR based devices depend on the coupling of an optical beam;which is incident upon a dielectric-metal structure placed in opticalproximity, to a surface plasmon-polariton mode supported by the metalstructure. At a specific angle of incidence, which depends on thematerials used and the particular geometry of the device, coupling to aplasmon mode is maximised and a drop in the power reflected from themetal surface is observed. ATR based modulators make use of thisattenuated reflection phenomenon along with means for varyingelectrically or otherwise at least one of the optical parameters of oneof the dielectrics bounding the metal structure in order to shift theangle of incidence where maximum coupling to plasmons occurs.Electrically shifting the angle of maximum coupling results in amodulation of the intensity of the reflected light. Examples of devicesthat are based on this architecture are disclosed in references [23] to[36].

[0015] Mode coupling devices are based on the optical coupling of lightpropagating in a dielectric waveguide to a nearby metal film placed acertain distance away and in parallel with the dielectric waveguide. Thecoupling coefficient between the optical mode propagating in thewaveguide and the plasmon-polariton mode supported by the nearby metalfilm is adjusted via the materials selected and the geometricalparameters of the device. Means is provided for varying, electrically orotherwise, at least one of the optical parameters of one of thedielectrics bounding the metal. Varying an optical parameter (the indexof refraction, say) varies the coupling coefficient between the opticalwave propagating in the dielectric waveguide and the lossyplasmon-polariton wave supported by the metal. This results in amodulation in the intensity of the light exiting the dielectricwaveguide. References [37] to [40] disclose various deviceimplementations based upon this phenomenon. Reference [41] furtherdiscusses the physical phenomenon underlying the operation of thesedevices.

[0016] Reference [42] discusses an application of the ATR phenomenon forrealising an optical switch or bistable device.

[0017] These known modulation and switching devices disadvantageouslyrequire relative high control voltages and have limitedelectrical/optical bandwidth.

SUMMARY OF THE INVENTION

[0018] The present invention seeks to eliminate, or at least mitigate,one or more of the disadvantages of the prior art, or at least providean alternative.

[0019] According to one aspect of the present invention there isprovided an optical device comprising a waveguide structure formed by athin strip of material having a relatively high free charge carrierdensity surrounded by material having a relatively low free chargecarrier density, the strip having finite width and thickness withdimensions such that optical radiation having a free space wavelength inthe range from about 0.81 μm to about 2 μm couples to the strip andpropagates along the length of the strip as a plasmon-polariton wave.

[0020] Such a strip of finite width offers two-dimensional (2-D)confinement in the transverse plane, i.e. perpendicular to the directionof propagation, and, since suitable low-loss waveguides can befabricated from such strip, it may be useful for signal transmission androuting or to construct components such as couplers, power splitters,interferometers, modulators, switches and other typical components ofintegrated optics. In such devices, different sections of the stripserving different functions, in some cases in combination withadditional electrodes. The strip sections may be discrete andconcatenated or otherwise interrelated, or sections of one or morecontinuous strips.

[0021] For example, where the optical radiation has a free-spacewavelength of 1550 nm, and the waveguide is made of a strip of a noblemetal surrounded by a good dielectric, say glass, suitable dimensionsfor the strip are thickness less than about 0.1 microns, preferablyabout 20 nm, and width of a few microns, preferably about 4 microns.

[0022] The strip could be straight, curved, bent, tapered, and so on.

[0023] The dielectric material may be inhomogeneous, for example acombination of slabs, strips, laminae, and so on. The conductive orsemiconductive strip may be inhomogeneous, for example a layer of goldand a layer of titanium.

[0024] The plasmon-polariton wave which propagates along the structuremay be excited by an appropriate optical field incident at one of theends of the waveguide, as in an end-fire configuration, and/or by adifferent radiation coupling means.

[0025] The device may further comprise means for varying the value of anelectromagnetic property of at least a portion of said surroundingmaterial so as to vary the propagation characteristics of the waveguidestructure and the propagation of the plasmon-polariton wave.

[0026] In some embodiments of the invention, for one said value of theelectromagnetic property, propagation of the plasmon-polariton wave issupported and, for another value of said electromagnetic property,propagation of the plasmon-polariton wave is at least inhibited. Suchembodiments may comprise modulators or switches.

[0027] Different embodiments of the invention may employ different meansof varying the electromagnetic property, such as varying the size of atleast one of said portions, especially if it comprises a fluid.

[0028] The at least one variable electromagnetic property of thematerial may comprise permittivity, permeability or conductivity.

[0029] Various objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription, taken in conjunction with the accompanying drawings, ofpreferred embodiments of the invention which are described by way ofexample only.

BRIEF DESCRIPTION OF DRAWINGS

[0030] FIGS. 1(a) and 1(b) are a cross-sectional illustration and a planview, respectively, of a symmetric waveguide structure embodying thepresent invention in which the core is comprised of a lossy metal filmof thickness t, width w, length l and permittivity ε₂ embedded in acladding or background comprising an “infinite” homogeneous dielectrichaving a permittivity ε₁;

[0031] FIGS. 2(a) and 2(b) illustrate dispersion characteristics withthickness of the first eight modes supported by a symmetric metal filmwaveguide of width w=1 μm. The a_(b) and s_(b) modes supported for thecase w=∞ are shown for comparison. (a) Normalized phase constant; (b)Normalized attenuation constant;

[0032] FIGS. 3(a),(b),(c),(d),(e) and (f) illustrate the spatialdistribution of the six field components related to the ss_(b) ⁰ modesupported by a symmetric metal film waveguide of thickness t=100 nm andwidth w=1 μm. The waveguide cross-section is located in the x-y planeand the metal is bounded by the region −0.5≦×≦0.5 μm and −0.05≦y≦0.05μm, outlined as the rectangular dashed contour. The field distributionsare normalized such that max|Re{E_(y)}|=1;

[0033] FIGS. 4(a),(b),(c),(d),(e) and (f) illustrate the spatialdistribution of the six field components related to the sa_(b) ⁰ modesupported by a symmetric metal film waveguide of thickness t=100 nm andwidth w=1 μm. The waveguide cross-section is located in the x-y planeand the metal is bounded by the region −0.5≦×≦0.5 μm and −0.05≦y≦0.05μm, outlined as the rectangular dashed contour. The field distributionsare normalized such that max|Re{E_(y)}|=1;

[0034] FIGS. 5(a),(b),(c),(d),(e) and (f) illustrate the spatialdistribution of the six field components related to the as_(b) ⁰ modesupported by a symmetric metal film waveguide of thickness t=100 nm andwidth w=1 μm. The waveguide cross-section is located in the x-y planeand the metal is bounded by the region −0.5≦×≦0.5 μm and −0.05≦y≦0.05μm, outlined as the rectangular dashed contour. The field distributionsare normalized such that max|Re{E_(y)}|=1;

[0035] FIGS. 6(a),(b),(c),(d),(e) and (f) illustrate the spatialdistribution of the six field components related to the aa_(b) ⁰ modesupported by a symmetric metal film waveguide of thickness t=100 nm andwidth w=1 μm. The waveguide cross-section is located in the x-y planeand the metal is bounded by the region −0.5≦×≦0.5 μm and −0.05≦y≦0.05μm, outlined as the rectangular dashed contour. The field distributionsare normalized such that max|Re{E_(y)}|=1;

[0036] FIGS. 7(a),(b),(c),(d),(e) and (f) are contour plots of Re{S_(z)}associated with the ss_(b) ⁰ mode for symmetric metal film waveguides ofwidth w=1 μm and various thicknesses. The power confinement factor cf isalso given in all cases, and is computed via equation (12) with the areaof the waveguide core A, taken as the area of the metal region. In allcases, the outline of the metal film is shown as the rectangular dashedcontour;

[0037]FIG. 8 illustrates a normalized profile of Re{S_(z)} associatedwith the ss_(b) ⁰ mode for a symmetric metal film waveguide of width w=1μm and thickness t=20 nm. The waveguide cross-section is located in thex-y plane and the metal film is bounded by the region −0.5≦×≦0.5 μm and−0.01≦y≦0.01 μm, outlined as the rectangular dashed contour;

[0038] FIGS. 9(a),(b),(c) and (d) illustrate the spatial distribution ofthe E_(y) field component related to some higher order modes supportedby a symmetric metal film waveguide of thickness t=100 nm and width w=1μm. In all cases, the waveguide cross-section is located in the x-yplane and the metal film is bounded by the region −0.5≦×≦0.5 μm and−0.05≦y≦0.05 μm, outlined as the rectangular dashed contour;

[0039] FIGS. 10(a) and (b) illustrate dispersion characteristics withthickness of the first six modes supported by a symmetric metal filmwaveguide of width w=0.5 μm. The a_(b) and s_(b) modes supported for thecase w=∞ are shown for comparison. (a) Normalized phase constant; (b)Normalized attenuation constant;

[0040] FIGS. 11(a) and (b) illustrate dispersion characteristics withthickness of the ss_(b) ⁰ mode supported by symmetric metal filmwaveguides of various widths. The s_(b) mode supported for the case w=∞is shown for comparison. (a) Normalized phase constant; (b) Normalizedattenuation constant;

[0041] FIGS. 12(a),(b),(c) and (d) illustrate a contour plot ofRe{S_(z)} associated with the ss_(b) ⁰ mode for symmetric metal filmwaveguides of thickness t=20 nm and various widths. The powerconfinement factor cf is also given in all cases, and is computed viaequation (12) with the area of the waveguide core A_(c) taken as thearea of the metal region. In all cases, the outline of the metal film isshown as the rectangular dashed contour;

[0042]FIG. 13 illustrates dispersion characteristics with thickness ofthe ss_(b) ⁰ mode supported by a symmetric metal film waveguide of widthw=0.5 μm for various background permittivities ε_(r,1*) . The normalizedphase constant is plotted on the left axis and the normalizedattenuation constant is plotted on the right one;

[0043] FIGS. 14(a),(b),(c) and (d) illustrates a contour plot of Re{Sz}associated with the ss_(b) ⁰ mode for a symmetric metal film waveguideof width w=0.5 μm and thickness t=20 nm for various backgroundpermittivities ε_(r,1*) . In all cases, the outline of the metal film isshown as the rectangular dashed contour;

[0044] FIGS. 15(a) and (b) illustrate dispersion characteristics withfrequency of the ss_(b) ⁰ mode supported by symmetric metal filmwaveguides of width w=0.5 μm and w=1 μm and various thicknesses t. Thes_(b) mode supported for the case w=∞ and the thicknesses considered isshown for comparison. (a) Normalized phase constant. (b) Mode powerattenuation computed using Equation (16) and scaled to dB/cm;

[0045] FIGS. 16(a),(b),(c),(d),(e) and (f) illustrate a contour plot ofRe{S_(z)} associated with the ss_(b) ⁰ mode for symmetric metal filmwaveguides of width w=0.5 μm and w=1 μm, and thickness t=20 nm atvarious free-space wavelengths of excitation λ₀. In all cases, theoutline of the metal film is shown as the rectangular dashed contour;

[0046] FIGS. 17(a) and 17(b) are a cross-sectional view and a plan view,respectively, of a second embodiment of the invention in the form of anasymmetric waveguide structure formed by a core comprising a lossy metalfilm of thickness t, width w and permittivity ε₂ supported by ahomogeneous semi-infinite substrate of permittivity ε₁ and with a coveror superstrate comprising a homogeneous semi-infinite dielectric ofpermittivity ε₃;

[0047] FIGS. 18(a) and 18(b) illustrate dispersion characteristics withthickness of the first seven modes supported by an asymmetric metal filmwaveguide of width w=1 μm. The a_(b) and s_(b) modes supported for thecase w=∞ are shown for comparison. (a) Normalized phase constant. (b)Normalized attenuation constant;

[0048] FIGS. 19(a),(b),(c) and (d) illustrate spatial distribution ofthe E_(y) field component related to the ss_(b) ⁰ mode supported by anasymmetric metal film waveguide of width w=1 μm for four filmthicknesses. The waveguide cross-section is located in the x-y plane andthe metal region is outlined as the rectangular dashed contour. Thefield distributions are normalized such that max|

{E_(y)}|=1;

[0049] FIGS. 20(a),(b),(c) and (d) illustrate spatial distribution ofthe E_(y) field component related to two higher order modes supported byan asymmetric metal film waveguide of width w=1 μm for two filmthicknesses. In all cases, the waveguide cross-section is located in thex-y plane and the metal region is outlined as the rectangular dashedcontour. The field distributions are normalized such that max|

{E_(y)}|=1;

[0050] FIGS. 21(a) and (b) illustrate dispersion characteristics withthickness of the first six modes supported by an asymmetric metal filmwaveguide of width w=1 μm. The a_(b) and s_(b) modes supported for thecase w=∞ are shown for comparison. (a) Normalized phase constant. (b)Normalized attenuation constant;

[0051] FIGS. 22(a),(b),(c) and (d) illustrate spatial distribution ofthe E_(y) field component related to modes supported by an asymmetricmetal film waveguide of width w=1 μm. In all cases, the waveguidecross-section is located in the x-y plane and the metal region isoutlined as the rectangular dashed contour. The field distributions arenormalized such that max|

{E_(y)}|=1;

[0052] FIGS. 23(a) and 23(b) illustrate dispersion characteristics withthickness of the first six modes supported by an asymmetric metal filmwaveguide of width w=0.5 μm. The a_(b) and s_(b) modes supported for thecase w=∞ are shown for comparison. (a) Normalized phase constant. (b)Normalized attenuation constant;

[0053] FIGS. 24(a) and 24(b) illustrate dispersion characteristics withthickness of the ss_(b) ⁰ and sa_(b) ¹ modes supported by an asymmetricmetal film waveguide of width w=0.5 μm for various cases of ε₃. (a)Normalized phase constant; the inset shows an enlarged view of theregion bounded by 0.04≦t≦0.08 μm and 2.0≦β/β₀≦2.3. (b) Normalizedattenuation constant; the inset shows an enlarged view of the regionbounded by 0.05≦t≦0.08 μm and 7.0×10⁻³≦α/β₀≦2.0×10⁻²;

[0054] FIGS. 25(a),(b),(c) and (d) illustrate spatial distribution ofthe E_(y) field component related to the sa_(b) ¹ mode supported by anasymmetric metal film waveguide of width w=0.5 μm for four filmthicknesses. The waveguide cross-section is located in the x-y plane andthe metal region is outlined as the rectangular dashed contour. Thefield distributions are normalized such that max|

{E_(y)}|=1;

[0055] FIGS. 26(a),(b),(c) and (d) illustrate a contour plot of

{S_(z)} associated with the long-ranging modes supported by asymmetricmetal film waveguides of width w=0.5 μm and having different superstratepermittivities ε₃. In all cases, the outline of the metal film is shownas the rectangular dashed contour;

[0056]FIG. 27 is a plan view of a waveguide with opposite sides steppedto provide different widths;

[0057]FIG. 28 is a plan view of a waveguide which is tapered andslanted;

[0058]FIG. 29 is a plan view of a trapezoidal waveguide;

[0059]FIG. 30 is a plan view of a waveguide having curved side edges andsuitable for use as a transition piece;

[0060]FIG. 31 is a plan view of a curved waveguide section suitable forinterconnecting waveguides at a corner;

[0061]FIG. 32 is a plan view of a two-way splitter/combiner formed by acombination of three straight waveguide sections and one taperedwaveguide section;

[0062]FIG. 33(a) is a plan view of an angled junction using a slantedsection;

[0063]FIG. 33(b) is a plan view of an offset junction using an S-bend;

[0064]FIG. 34(a) is a plan view of a power divider formed by atrapezoidal section and pairs of concatenated bends;

[0065]FIG. 34(b) is a plan view of a power divider similar to that shownin FIG. 34(a) but with a transition section formed by mirroring andoverlapping curved sections;

[0066]FIG. 35 is a plan view of a Mach-Zender interferometer formedusing a combination of the waveguide sections;

[0067]FIG. 36(a) is a schematic plan view of a modulator using theMach-Zender waveguide structure of FIG. 35;

[0068] FIGS. 36(b) and 36(c) are inset diagrams illustrating alternativeways of applying a modulation control voltage;

[0069]FIG. 37 is a plan view of a modulator using the Mach-Zenderwaveguide structure of FIG. 35 and illustrating magnetic field control;

[0070]FIG. 38 is a plan view of a periodic structure formed by a seriesof unit cells each comprising two waveguide sections having differentwidths and lengths;

[0071]FIG. 39 is a plan view of a periodic waveguide structure formed bya series of unit cells each comprising two opposed trapezoidal waveguidesections;

[0072]FIG. 40(a) is a plan view of an edge coupler formed by twoparallel strips of straight waveguide with various other waveguides forcoupling signals to and from them;

[0073]FIG. 40(b) is an inset diagram illustrating a way of applying amodulation control voltage;

[0074]FIG. 40(c) is a plan view of an edge coupler similar to that shownin FIG. 40(a) but using S-bends;

[0075]FIG. 41(a) is a perspective view of an edge coupler in which theparallel strips are not co-planar;

[0076]FIG. 41(b) is an inset diagram illustrating a way of applying amodulation control voltage;

[0077]FIG. 42 is a plan view of an intersection formed by four sectionsof waveguide;

[0078] FIGS. 43(a) and 43(b) are a schematic front view andcorresponding top plan view of an electro-optic modulator employing thewaveguide structure of FIG. 17(a);

[0079] FIGS. 44(a) and 44(b) are a schematic front view andcorresponding top view of an alternative electro-optic modulator alsousing the waveguide structure of FIG. 17(a);

[0080]FIG. 44(c) illustrates an alternative connection arrangement ofthe modulator of FIG. 44(a);

[0081]FIG. 45 is a schematic front view of a third embodiment ofelectro-optic modulator also using the waveguide structure of FIG.17(a);

[0082]FIG. 46 is a schematic front view of a magneto-optic modulatoralso using the waveguide structure of FIG. 17(a);

[0083]FIG. 47 is a schematic front view of a thermo-optic modulator alsousing the waveguide structure of FIG. 17(a);

[0084]FIG. 48 is a schematic perspective view of an electro-optic switchalso using the waveguide structure of FIG. 17(a);

[0085]FIG. 49 is a schematic perspective view of a magneto-optic switchalso using the waveguide structure of FIG. 17(a);

[0086]FIG. 50 is a schematic perspective view of a thermo-optic switchalso using the waveguide structure of FIG. 17(a);

[0087]FIG. 51 gives the mode power attenuation for metal film waveguidesof various widths and thicknesses. The metal used is Au and thebackground dielectric is SiO₂. The optical free-space wavelength ofanalysis is set to λ₀=1.55 μm; and

[0088]FIG. 52 gives the mode power attenuation for metal film waveguidesof various widths and thicknesses. The metal used is Al and thebackground dielectric is SiO₂. The optical free-space wavelength ofanalysis is set to λ₀=1.55 μm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0089] I. Introduction

[0090] In order to facilitate an understanding of the specific opticaldevices embodying the invention, their theoretical basis will first beexplained with reference to FIGS. 1 to 26(d).

[0091] The following is a comprehensive description of the purely boundmodes of propagation supported by symmetric and asymmetric waveguidestructures comprised of a thin lossy metal film of finite-width as thecore. The core can be embedded in an “infinite” homogeneous dielectricmedium as shown in FIG. 1(a) or supported by a semi-infinite homogeneousdielectric substrate and covered by a different semi-infinitehomogeneous dielectric superstrate as shown in FIG. 17(a). Thedescription is organized as follows. Section II summarizes the physicalbasis and numerical technique used to analyze the structures ofinterest. Sections III through-VI describe the modes supported bysymmetric structures as shown in FIG. 1(a) and sections VII through Xdescribe the modes supported by asymmetric structures as shown in FIG.17(a). Concluding remarks are given in section XI.

[0092] II. Physical Basis and Numerical Technique

[0093] A symmetric structure embodying the present invention is shown inFIGS. 1(a) and 1(b). It comprises a lossy metal film of thickness t,width w and equivalent permittivity ε₂, surrounded by a cladding orbackground comprising an infinite homogeneous dielectric of permittivityε₁. FIG. 17(a) shows an asymmetric structure (ε₁≠ε₃) embodying thepresent invention. The Cartesian coordinate axes used for the analysisare also shown with propagation taking place along the z axis, which isout of the page.

[0094] It is assumed that the metal region shown in FIGS. 1(a) and 17(a)can be modelled as an electron gas over the wavelengths of interest.According to classical or Drude electron theory, the complex relativepermittivity of the metal region is given by the well-known plasmafrequency dispersion relation [4]: $\begin{matrix}{ɛ_{r,2} = {\left( {1 - \frac{\omega_{p}^{2}}{\omega^{2} + v^{2}}} \right) - {j\left( \frac{\omega_{p}^{2}v}{\omega\left( {\omega^{2} + v^{2}} \right.} \right)}}} & (1)\end{matrix}$

[0095] where ω is the excitation frequency, ω_(p) is the electron plasmafrequency and ν is the effective electron collision frequency, oftenexpressed as ν=1/τ with τ defined as the relaxation time of electrons inthe metal. When ω²+ν²<ω_(p) ² (which is the case for many metals atoptical wavelengths) a negative value for the real part ε_(r,2) isobtained, implying that plasmon-polariton modes can be supported atinterfaces with normal dielectrics.

[0096] Electromagnetic Wave and Field Equations

[0097] The modes supported by the structures are obtained by solving asuitably defined boundary value problem based on Maxwell's equationswritten in the frequency domain for a lossy inhomogeneous isotropicmedium. Uncoupling Maxwell's equations yields the followingtime-harmonic vectorial wave equations for the E and H fields:

∇×∇×E−ω ²ε(x,y)μE=0   (2)

∇×ε(x,y)⁻¹ ∇×H−ω ² μH=0   (3)

[0098] where the permittivity ε is a complex function of cross-sectionalspace, and describes the waveguide structure. For the structuresanalyzed in this description, μ is homogeneous and taken as thepermeability of free space μ₀.

[0099] Due to the nature of the numerical method used to solve theboundary value problem, the implicit y dependence of the permittivitycan be immediately removed since any inhomogeneity along y is treated bydividing the structure into a number of layers that are homogeneousalong this direction, and suitable boundary conditions are appliedbetween them.

[0100] The two vectorial wave equations (2) and (3) are expanded in eachlayer into scalar wave equations, some being coupled by virtue of theremaining inhomogeneity in ε along x. Since the structure underconsideration is invariant along the propagation axis (taken to be inthe +z direction), the mode fields vary along this dimension accordingto e^(−γz) where γ=α+jβ is the complex propagation constant of the mode,α being its attenuation constant and β its phase constant. Substitutingthis field dependency into the scalar wave equations, and writing themfor TE^(x)(E_(x)=0) and TM^(x)(H_(x)=0) modes while making use of∇·[ε(x)E]=0 and ∇·H=0 accordingly, yields simplified and uncoupledscalar wave equations that are readily solved. The E_(y) component ofthe TE^(x) modes must satisfy the Helmholtz wave equation:$\begin{matrix}{{{\frac{\partial^{2}}{\partial x^{2}}E_{y}^{TE}} + {\frac{\partial^{2}}{\partial y^{2}}E_{y}^{TE}} + {\left\lbrack {\gamma^{2} + {\omega^{2}\mu \quad {\varepsilon (x)}}} \right\rbrack E_{y}^{TE}}} = 0} & (4)\end{matrix}$

[0101] and the H_(y) component of the TM^(x) modes must satisfy theSturm-Liouville wave equation: $\begin{matrix}{{{{\varepsilon (x)}{\frac{\partial}{\partial x}\left\lbrack {\frac{1}{\varepsilon (x)}\frac{\partial}{\partial x}H_{y}^{TM}} \right\rbrack}} + {\frac{\partial^{2}}{\partial y^{2}}H_{y}^{TM}} + {\left\lbrack {\gamma^{2} = {\omega^{2}\mu \quad {\varepsilon (x)}}} \right\rbrack H_{y}^{TM}}} = 0} & (5)\end{matrix}$

[0102] The superposition of the TE^(x) and TM^(x) mode families thendescribes any mode propagating in the structure analyzed. The electricand magnetic field components resulting from this superposition aregiven by the following equations: $\begin{matrix}{E_{x} = {\frac{- 1}{j\quad \omega \quad \gamma}\left\lbrack {{\frac{\partial}{\partial x}\left( {\frac{1}{\varepsilon (x)}\frac{\partial}{\partial x}H_{y}^{TM}} \right)} + {\omega^{2}\mu \quad H_{y}^{TM}}} \right\rbrack}} & (6) \\{E_{y} = {E_{y}^{TE} - {\frac{}{j\quad \omega \quad \gamma \quad {\varepsilon (x)}}\frac{\partial^{2}}{{\partial x}{\partial y}}H_{y}^{TM}}}} & (7) \\{E_{z} = {{\frac{1}{\gamma}\frac{\partial}{\partial y}E_{y}^{TE}} + {\frac{1}{j\quad \omega \quad {\varepsilon (x)}}\frac{\partial}{\partial x}H_{y}^{TM}}}} & (8) \\{H_{x} = {\frac{1}{j\quad \omega \quad \gamma}\left\lbrack {{\frac{1}{\mu}\frac{\partial^{2}}{\partial x^{2}}E_{y}^{TE}} + {\omega^{2}{\varepsilon (x)}E_{y}^{TE}}} \right\rbrack}} & (9) \\{H_{y} = {{\frac{1}{j\quad \omega \quad \gamma \quad \mu}\frac{\partial^{2}}{{\partial x}{\partial y}}E_{y}^{TE}} + H_{y}^{TM}}} & (10) \\{H_{z} = {{{- \frac{1}{j\quad \omega \quad \mu}}\frac{\partial}{\partial x}E_{y}^{TE}} + {\frac{1}{\gamma}\frac{\partial}{\partial y}H_{y}^{TM}}}} & (11)\end{matrix}$

[0103] In order to obtain a mode of propagation supported by awaveguiding structure, the Helmholtz and Sturm-Liouville wave equations(4) and (5), along with the field equations (6)-(11), must be solved forthe propagation constant γ using appropriate boundary conditions appliedbetween layers and at the horizontal and vertical limits.

[0104] Poynting Vector and Power Confinement Factor

[0105] The power confinement factor is defined as the ratio of modecomplex power carried through a portion of a waveguide's cross-sectionwith respect to the mode complex power carried through the entirewaveguide cross-section. Formally it is expressed as: $\begin{matrix}{{cf} = \frac{{\int{\int_{A_{c}}{S_{z}{s}}}}}{{\int{\int_{A_{\infty}}{s_{z}{s}}}}}} & (12)\end{matrix}$

[0106] where A_(c) is usually taken as the area of the waveguide coreand A_(∞) implies integration over the entire waveguide cross-section(which can be all cross-sectional space for an open structure) or theentire cross-sectional computational domain. S_(z) refers to the zcomponent of the Poynting vector: $\begin{matrix}{S_{z} = {\frac{1}{2}\left( {{E_{x}H_{y}^{*}} - {E_{y}H_{x}^{*}}} \right)}} & (13)\end{matrix}$

[0107] and H_(x,y)* denotes the complex conjugate of H_(x,y*) . Thespatial distribution of a component of the Poynting vector is easilycomputed from the spatial distribution of the relevant electric andmagnetic mode field components.

[0108] Numerical Solution Approach

[0109] The boundary value problem governed by equations (4) to (11) issolved by applying the Method of Lines (MoL). The MoL is a well-knownnumerical technique and its application to various electromagneticproblems, including optical waveguiding, is well-established [14]. TheMoL is rigorous, accurate and flexible. It can handle a wide variety ofwaveguide geometries, including the structures at hand. The method isnot known to generate spurious or non-physical modes. The MoLformulation used herein is based on the formulation reported in [15],but simplified for isotropic media, as prescribed by equations (4)-(11)and reported in [16]. Except for a 1-D spatial discretization, themethod is exact.

[0110] The main idea behind the MoL is that the differential fieldequations governing a waveguiding problem are discretized only as far asnecessary so that generalized analytic solutions can be applied toderive a homogeneous matrix problem describing all modes supported bythe structure. This approach renders the method accurate andcomputationally efficient since only −1 dimensions must be discretizedto solve an N dimension problem. In the case of a two-dimensional (2-D)waveguiding structure, this means that only one spatial dimension needsto be discretized. The main features of this procedure, as applied to amodal analysis problem, are described below.

[0111] The x axis and the function ε(x) are discretized using twoshifted non-equidistant line systems, parallel to the y axis.

[0112] The differential operators ∂\∂x and ∂²/∂²x in the wave and fieldequations are replaced by finite difference approximations that includethe lateral boundary conditions.

[0113] The discretized wave equations are diagonalized using appropriatetransformations matrices.

[0114] The diagonalization procedure yields in the transform domain twosystems of uncoupled one-dimensional (1-D) differential equations alongthe remaining dimension (in this case along the y axis).

[0115] These differential equations are solved analytically andtangential field matching conditions are applied at interfaces betweenlayers along with the top and bottom boundary conditions.

[0116] The last field matching condition, applied near the center of thestructure, yields a homogeneous matrix equation of the form G(γ){tildeover (e)}=0 which operates on transformed tangential fields.

[0117] The complex propagation constant γ of modes is then obtained bysearching for values that satisfy det[G(γ)]=0.

[0118] Once the propagation constant of a mode has been determined, thespatial distribution of all six field components of the mode are easilygenerated.

[0119] A mode power confinement factor can be computed by firstcomputing the spatial distribution of S_(z) which is then integratedaccording to Equation (12).

[0120] The open structures shown in FIGS. 1(a) and 17(a) are discretizedalong the x axis and the generalized analytic solution applied along they axis. The physical symmetry of the structures is exploited to increasethe accuracy of the results and to reduce the numerical effort requiredto generate the mode solutions. For the symmetric structure shown inFIG. 1(a), this is achieved by placing either electric wall (E_(tan)=0)or magnetic wall (H_(tan)=0) boundary conditions along the x and y axes.For the asymmetric structure shown in FIG. 17(a), this is achieved byplacing electric wall or magnetic wall boundary conditions along the yaxis only. The remaining horizontal boundary conditions are placed atinfinity and the remaining lateral boundary condition is either placedfar enough from the guide to have a negligible effect on the modecalculation, or a lateral absorbing boundary condition is used tosimulate infinite space, depending on the level of confinement observedin the resulting mode. The use of numerical methods to solvedifferential equations inevitably raises questions regarding theconvergence of computed results and their accuracy. The propagationconstant of a mode computed using the method of lines converges in amonotonic or smooth manner with a reduction in the discretizationinterval (which increases the number of lines in the calculation andthus the numerical effort). This suggests that extrapolation can be usedto generate a more accurate value for the propagation constant, and thisvalue can then be used to compute the error in values obtained using thecoarser discretizations [17]. This anticipated error does not correspondto the actual error in the propagation constant as the latter could onlybe known if the analytic or exact value were available. The anticipatederror, however, still provides a useful measure of accuracy since itmust tend toward zero as more accurate results are generated.

[0121] The convergence of the computed propagation constant of the modessupported by the structures of interest has been monitored during theentire study. The anticipated error in the results presented herein isestimated as 1% on average and 6% in the worst case. These error valuesare based on extrapolated propagation constants computed usingRichardson's extrapolation formula [18].

[0122] III. Mode Characteristics and Evolution With Film Thickness:Symmetric Structures

[0123] A. Review of Mode Solutions for Metal Film Slab Waveguides

[0124] The review begins with the reproduction of results for aninfinitely wide symmetric metal film waveguide, as shown in FIG. 1(a)with w=∞, taken from the standard work on such structures [6]. In orderto remain consistent with their results, the optical free-spacewavelength of excitation is set to λ₀=0.633 μm and their value for therelative permittivity of the silver film at this wavelength is used:ε_(r,2)=−19−j0.53. The relative permittivity of the top and bottomdielectric regions is set to ε_(r,1)=4.

[0125] An infinitely wide structure supports only two purely bound TM(E_(x)=H_(y)=H_(z)=0) modes having transverse field components E_(y) andH_(x) that exhibit asymmetry or symmetry with respect to the x axis.These modes are created from the coupling of individualplasmon-polariton modes supported by the top and bottom interfaces andthey exhibit dispersion with film thickness. The widely acceptednomenclature for identifying them consists in using the letters a or sfor asymmetric or symmetric transverse field distributions,respectively, followed by a subscript b or l for bound or leaky modes,respectively. The propagation constants of the a_(b) and s_(b) modeshave been computed as a function of film thickness and the normalizedphase and attenuation constants are plotted in FIGS. 2(a) and 2(b),respectively.

[0126] From FIGS. 2(a) and 2(b), it is observed that the a_(b) and s_(b)modes become degenerate with increasing film thickness. As theseparation between the top and bottom interfaces increases, the a_(b)and s_(b) modes begin to split into a pair of uncoupledplasmon-polariton modes localized at the metal-dielectric interfaces.The propagation constants of the a_(b) and s_(b) modes thus tend towardsthat of a plasmon-polariton mode supported by the interface betweensemi-infinite metallic and dielectric regions, which is given via thefollowing equations [6]: $\begin{matrix}{{\beta/\beta_{0}} = {{- {Re}}\left\{ \sqrt{\frac{\varepsilon_{r,1}\varepsilon_{r,2}}{\varepsilon_{r,1} + \varepsilon_{r,2}}} \right\}}} & (14) \\{{\alpha/\beta_{0}} = {{- {Im}}\left\{ \sqrt{\frac{\varepsilon_{r,1}\varepsilon_{r,2}}{\varepsilon_{r,1} + \varepsilon_{r,2}}} \right\}}} & (15)\end{matrix}$

[0127] where β₀=ω/c₀ with c₀ being the velocity of light in free space,and ε_(r,1) and ε_(r,2) are the complex relative permittivities of thematerials used. Using the above equations, values of β/β₀=2.250646 andα/β₀=0.836247×10⁻² are obtained for ε_(r,1)=4 and ε_(r,2)=−19−j0.53.

[0128] As the thickness of the film decreases, the phase and attenuationconstants of the a_(b) mode increase, becoming very large for very thinfilms. This is due to the fact that the fields of this mode penetrateprogressively deeper into the metal as its thickness is reduced. In thecase of the s_(b) mode, a decreasing film thickness causes the oppositeeffect, that is, the fields penetrate progressively more into the topand bottom dielectric regions and less into the metal. The propagationconstant of this mode thus tends asymptotically towards that of a TEM(Transverse ElectroMagnetic) wave propagating in an infinite mediumhaving the same permittivity as the top and bottom dielectric regions.In this case, the attenuation constant decreases asymptotically towardszero since losses were neglected in these regions. The a_(b) and s_(b)modes do not have a cutoff thickness.

[0129] The fields in an infinitely wide structure do not exhibit anyspatial variation along x. Due to the nature of the MoL, and to the factthat the generalized analytical solution is applied along the ydimension, our results do not contain discretization errors and thus arein perfect agreement with those reported in [6].

[0130] B. Modes Supported by a Metal Film of Width w=1 μm

[0131] Next, the analysis of the structure shown in FIG. 1(a) for thecase w=1 μm will be explained. The material parameters and free-spacewavelength that were used in the previous case (w=∞) were also usedhere. The MoL was applied and the discretization adjusted untilconvergence of the propagation constant was observed. The physicalquarter-symmetry of the structure was exploited by placing vertical andhorizontal electric or magnetic walls along the y and x axes,respectively, which leads to four possible wall combinations as listedin Table 1. The first two purely bound (non-leaky) modes for each wallcombination were found and their dispersion with metal thicknesscomputed. The results for these eight modes are shown in FIGS. 2(a) and2(b). TABLE 1 Vertical-Horizontal wall combinations used along the axesof symmetry and proposed mode nomenclature: ew - electric wall, mw -magnetic wall. V-H Walls Mode ew—ew as_(b) ^(m) mw-ew ss_(b) ^(m) mw—mwsa_(b) ^(m) ew-mw aa_(b) ^(m)

[0132] Unlike its slab counterpart, pure TM modes are not supported by ametal film of finite width: all six field components are present in allmodes. For a symmetric structure having an aspect ratio w/t>1, the Eyfield component dominates. The E_(x) field component increases inmagnitude with increasing film thickness and if w/t<1, then E_(x)dominates. It is proposed to identify the modes supported by a metalfilm of finite width, by extending the nomenclature used for metal filmslab waveguides. First a pair of letters being a or s identify whetherthe main transverse electric field component is asymmetric or symmetricwith respect to the y and x axes, respectively (in most practicalstructures w/t>>1 and E_(y) is the main transverse electric fieldcomponent). A superscript is then used to track the number of extremaobserved in the spatial distribution of this field component along thelargest dimension (usually along the x axis) between the corners. Asecond superscript n could be added to track the extrema along the otherdimension (the y axis) if modes exhibiting them are found. Finally, asubscript b or l is used to identify whether the mode is bound or leaky.Leaky modes are known to exist in metal film slab structures and thougha search for them has not been made at this time, their existence isanticipated. Table 1 relates the proposed mode nomenclature to thecorresponding vertical and horizontal wall combinations used along theaxes of symmetry.

[0133] The ss_(b) ⁰, sa_(b) ⁰, as_(b) ⁰ and aa_(b) ⁰ modes are the firstmodes generated (one for each of the four possible quarter-symmetrieslisted in Table 1, and having the largest phase constant) and thus maybe considered as the fundamental modes supported by the structure. FIGS.3 to 6 show the field distributions of these modes over thecross-section of the waveguide for a metal film of thickness t=100 nm.As is observed from these figures, the main transverse electric fieldcomponent is the E_(y) component and the symmetries in the spatialdistribution of this component are reflected in the mode nomenclature.The outline of the metal is clearly seen in the distribution of theE_(y) component on all of these plots. As is observed from the figures,very little field tunnels through the metal to couple parallel edges forthis case of film thickness and width (very little coupling through themetal between the top and bottom edges and between the left and rightedges), though coupling does occur along all edges between adjacentcorners (mostly along the left and right ones), and also betweenperpendicular edges through the corner.

[0134] FIGS. 2(a) and 2(b) suggests that the dispersion curves for thesefirst four modes converge with increasing film thickness toward thepropagation constant of a plasmon-polariton mode supported by anisolated corner (though pairs of corners in this case remain weaklycoupled along the top and bottom edges due to the finite width of thefilm, even if its thickness goes to infinity). If both the filmthickness and width were to increase further, the four fundamental modeswould approach degeneracy with their propagation constant tendingtowards that of a plasmon-polariton mode supported by an isolatedcorner, and their mode fields becoming more localized near the cornersof the structure with maxima occurring at all four corners and fieldsdecaying in an exponential-like manner in all directions away from thecorners. This is further supported by considering the evolution of thefield distributions given in FIGS. 3 to 6 as both the thickness andwidth increase.

[0135] As the thickness of the film decreases, coupling between the topand bottom edges increases and the four modes split into a pair as theupper branch (modes sa_(b) ⁰ and aa_(b) ⁰ which have a dominant E_(y)field component exhibiting asymmetry with respect to the x axis) and apair as the lower branch (modes ss_(b) ⁰ and as_(b) ⁰ which have adominant E_(y) field component exhibiting symmetry with respect to the xaxis), as shown in FIGS. 2(a) and 2(b). The pair on the upper branchremain approximately degenerate for all film thicknesses, thoughdecreasing the film width would eventually break this degeneracy. Theupper branch modes do not change in character as the film thicknessdecreases. Their field distributions remain essentially unchanged fromthose shown in FIGS. 4 and 6 with the exception that confinement to themetal region is increased thus causing an increase in their attenuationconstant. This field behaviour is consistent with that of the a_(b) modesupported by a metal film slab waveguide.

[0136] The modes on the lower branch begin to split at a film thicknessof about 80 nm, as shown in FIGS. 2(a) and 2(b). As the film thicknessdecreases further the ss_(b) ⁰ mode follows closely the phase andattenuation curves of the s_(b) mode supported by the metal film slabwaveguide. In addition to exhibiting dispersion, the lower branch modeschange in character with decreasing thickness, their fields evolvingfrom being concentrated near the corners, to having Gaussian-likedistributions along the waveguide width. The E_(y) field component ofthe ss_(b) ⁰ mode develops an extremum near the center of the top andbottom interfaces, while that of the as_(b) ⁰ mode develops two extrema,one on either side of the center. Since these modes change in character,they should be identified when the film is fairly thick.

[0137] FIGS. 7(a) to 7(f) show the evolution of the ss_(b) ⁰ mode fieldswith film thickness via contour plots of Re{S_(z)}. S_(z) is computedfrom the ss_(b) ⁰ mode fields using Equation 13 and corresponds to thecomplex power density carried by the mode. The power confinement factorcf is also given in the figure for all cases, and is computed viaequation (12) with the area of the waveguide core A_(c) taken as thearea of the metal region. FIGS. 7(a) to 7(f) clearly show how the modefields evolve from being confined to the corners of thick films to beingdistributed in a Gaussian-like manner laterally along the top and bottomedges, as the field coupling between these edges increases due to areduction in film thickness. The confinement factor becomes smaller asthe film thickness decreases, ranging from 14% confinement to 1.6% asthe thickness goes from 80 nm to 20 nm. This implies that fields becomeless confined to the metal, spreading out not only along the verticaldimension but along the horizontal one as well, as is observed bycomparing FIGS. 7(a) and 7(b). This reduction in confinement to thelossy metal region explains the reduction in the attenuation constant ofthe mode with decreasing film thickness, as shown in FIG. 2(b). Anexamination of all field components related to the ss_(b) ⁰ mode revealsthat the magnitudes of the weak transverse (E_(x), H_(y)) andlongitudinal (E_(z), H_(z)) components decrease with decreasing filmthickness, implying that the mode is evolving towards a TEM modecomprised of the E_(y) and H_(x) field components. Indeed, thenormalized propagation constant of the ss_(b) ⁰ mode tendsasymptotically towards the value of the normalized propagation constantof a TEM wave propagating in the background material (ε_(r,1)=4 with nolosses in this case), further supporting this fact. This field behaviouris also consistent with that of the s_(b) mode supported by a metal filmslab waveguide.

[0138]FIG. 8 shows the profile of Re{S_(z)} of the ss_(b) ⁰ mode overthe cross-section of the guide for the case t=20 nm, providing adifferent perspective of the same information plotted as contours inFIG. 7(f). FIG. 8 shows that Re{S_(z)} is negative in the metal film,implying that the mode real power is flowing in the direction oppositeto the direction of mode propagation (or to the direction of phasevelocity) in this region. It is clear however that the overall or netmode real power is flowing along the direction of propagation. It ispossible that the net mode real power can be made to flow in thedirection opposite to that of phase velocity (as in metal film slabwaveguides [10]) for values of ε_(r,1) in the neighbourhood or greaterthan |Re{ε_(r,2)}|.

[0139] Unlike the metal film slab waveguide, a metal film of finitewidth can support a number of higher order modes. The dispersion curvesof the first four higher order modes (each generated from one of thesymmetries listed in Table 1 are shown in FIGS. 2(a) and 2(b), and thespatial distribution of their main transverse electric field componentis shown in FIG. 9 for a film of thickness t=100 nm. As is observed fromFIGS. 9(a) to 9(d), the symmetries and number of extrema in thedistributions of Re{E_(y)} are reflected in the mode nomenclature. Itshould be noted that the nature of the nomenclature is such that allhigher order modes sa_(b) ^(m) and ss_(b) ^(m) have an odd m while allhigher order modes aa_(b) ^(m) and as_(b) ^(m) have an even m. ComparingFIGS. 9(a) to 9(d) with FIGS. 3(c), 4(c), 5(c) and 6(c), respectively,(ie: comparing the E_(y) component of the ss_(b) ¹ mode shown in FIG.9(a) with the E_(y) component of the ss_(b) ⁰ mode shown in FIG. 3(c),etc . . . ) reveals that the fields of a higher order mode are comprisedof the fields of the corresponding m=0 mode with additional spatialoscillations or variations along the top and bottom edges of thestructure due to the latter's limited width. Making this comparison forall of the field components of the higher order modes found reveals thisfact to be true, except for the H_(y) field component which remains inall cases essentially identical to that of the corresponding m=0 mode;ie: the H_(y) field component never exhibits oscillations along thewidth of the structure.

[0140] The evolution of the sa_(b) ¹ and aa_(b) ² modes with filmthickness is similar to the evolution of the sa_(b) and aa_(b) ⁰ modes(and the a_(b) mode supported by the metal film slab waveguide), in thattheir mode fields become more tightly confined to the metal as thethickness of the latter decreases, thereby causing an increase in theattenuation of the modes, as shown in FIG. 2(b). Furthermore, the sa_(b)¹ and aa_(b) ² modes do not change in character with film thickness,their field distributions remaining essentially unchanged in appearancefrom those computed at a thickness of 100 nm.

[0141] The ss_(b) ¹ and as_(b) ² modes evolve with thickness in a mannersimilar to the corresponding m=0 modes (and the s_(b) mode of the metalfilm slab waveguide) in the sense that their fields become less confinedto the metal region as the thickness of the latter decreases, therebyreducing the attenuation of the modes as shown in FIG. 2(b). As thethickness of the film decreases, the ss_(b) ¹ and as_(b) ² modes changein character in a manner similar to the corresponding m=0 modes, theirfield components evolving extra variations along the top and bottomedges.

[0142] As the thickness of the film increases, the propagation constantsof the sa_(b) ¹ and ss_(b) ¹ modes converge to a single complex value asshown in FIGS. 2(a) and 2(b). This is the propagation constant ofuncoupled higher order modes supported by the top and bottom edges ofthe film. A similar observation holds for the aa_(b) ² and as_(b) ²modes. The nature of these ‘edge modes’ is clear by considering theevolution with increasing film thickness of the distributions shown inFIGS. 9(a) to 9(d). As the thickness of the film tends to infinity, thetop edge becomes uncoupled from the bottom edge, forcing the ss_(b) ¹mode to become degenerate with the sa_(b) ¹ mode since both have anE_(y) field component that is symmetric with respect to the y axis andone extremum in its distribution along the top or bottom edge. A similarreasoning explains why the as_(b) ² mode must become degenerate with thess_(b) ² mode. In general, it is expected that the higher order sa_(b)^(m) and ss_(b) ^(m) mode families will form degenerate pairs for agiven m, as will the higher order as_(b) ^(m) and aa_(b) ^(m) modefamilies, with increasing film thickness.

[0143] The aa_(b) ^(m) and sa_(b) ^(m) mode families do not have modecutoff thicknesses. This is due to the fact that their confinement tothe metal film increases with decreasing film thickness; thus the modesremain guided as t→0. The as_(b) ^(m) and ss_(b) ^(m) mode families havecutoff thicknesses for all modes except the ss_(b) ⁰ mode, which remainsguided as t→0, since it evolves into the TEM mode supported by thebackground. The other modes of these families, including the as_(b) ⁰mode cannot propagate as t→0 because their mode fields do not evolveinto a TEM mode. Rather, the modes maintain extrema in their fielddistributions and such variations cannot be enforced by an infinitehomogeneous medium.

[0144] In general, the purely bound modes supported by a metal film offinite width appear to be formed from a coupling of modes supported byeach metal-dielectric interface defining the structure. In a metal filmof finite width, straight interfaces of finite length (top, bottom, leftand right edges) and corner interfaces are present. Since a straightmetal-dielectric interface of infinite length can support a boundplasmon-polariton mode then so should an isolated corner interface and astraight interface of finite length bounded by corners (say the edgedefined by a metal of finite width having an infinite thickness). Apreliminary analysis of an isolated corner has revealed that aplasmon-polariton mode is indeed supported and that the phase andattenuation constants of this mode are greater than those of the modeguided by the corresponding infinite straight interface, as given byEquations (14) and (15). This is due to the fact that fields penetratemore deeply into the metal near the corner, to couple neighbouringperpendicular edges. All six field components are present in such amode, having their maximum value at the corner and decreasing in anexponential-like manner in all directions away from the corner. Astraight interface of finite length bounded by corners should support adiscrete spectrum of plasmon-polariton modes with the defining featurein the mode fields being the number of extrema in their spatialdistribution along the edge. A mode supported by a metal film of finitewidth: may therefore be seen as being comprised of coupled ‘cornermodes’ and ‘finite length edge modes’.

[0145] The ss_(b) ⁰ mode could be used for optical signal transmissionover short distances. Its losses decrease with decreasing film thicknessin a manner similar to the s_(b) mode supported by the metal film slabwaveguide. In a symmetric waveguide structure such as the one studiedhere, the ss_(b) ⁰ mode does not have a cut-off thickness so lossescould be made small enough to render it long-ranging, though a trade-offagainst confinement is necessary. In addition, when the metal is thin,the E_(y) field component of the mode has a maximum near the center ofthe metal-dielectric interfaces, with a symmetric profile similar tothat shown in FIG. 8. This suggests that the mode should be excitableusing a simple end-fire technique similar to the one employed to excitesurface plasmon-polariton modes [19,6]; this technique is based onmaximizing the overlap between the incident field and that of the modeto be excited.

[0146] In reference [22], the present inventor et al. disclosed thatplasmon-polariton waves supported by thin metal films of finite widthhave recently been observed experimentally at optical communicationswavelengths using this method of excitation.

[0147] IV. Mode Dispersion With Film Width: Symmetric Structures

[0148] Since the modes supported by a metal film waveguide exhibitdispersion with film thickness, it is expected that they also exhibitdispersion with film width.

[0149] A. Modes Supported by a Metal Film of Width w=0.51 μm

[0150] The analysis of a metal film waveguide of width w=0.5 μm will nowbe discussed, using the material parameters and free-space wavelengththat were used in the previous section. A film width of 0.5 μm wasselected in order to determine the impact of a narrowing film on themodes supported and to demonstrate that the structure can still functionas a waveguide though the free-space optical wavelength is greater thanboth the width and thickness of the film.

[0151] As in the previous section, the first eight modes supported bythe structure (two for each symmetry listed in Table 1) were sought, butin this case only six modes were found. The dispersion curves withthickness of the modes found are plotted in FIGS. 10(a) and 10(b). Theobservations made in the previous section regarding the generalbehaviour of the modes hold for other film widths, including this one.

[0152] The aa_(b) ² and as_(b) ² modes, which were the highest ordermodes found for a film of width w=1 μm, were not found in this casesuggesting that the higher order modes (m>0) in general have a cut-offwidth. Comparing FIG. 10(a) with FIG. 2(a), it is apparent thatdecreasing the film width causes a decrease in the phase constant of thess_(b) ¹ and sa_(b) ¹ modes, further supporting the existence of acut-off width for these modes.

[0153] Comparing FIGS. 10(a) and 10(b) with FIGS. 2(a) and 2(b), it isnoted that the modes which do exhibit cutoff thicknesses (the ss_(b)^(m) modes with m>0 and the as_(b) ^(m) modes with m≧0), exhibit them ata larger thickness for a narrower film width. This makes it possible todesign a waveguide supporting only one long-ranging mode (the ss_(b) ⁰mode) by carefully selecting the film width and thickness.

[0154] B. Dispersion of the ss_(b) ⁰ Mode With Film Width

[0155] The dispersion with thickness of the ss_(b) ⁰ mode is shown inFIGS. 11(a) and 11(b) for numerous film widths in the range 0.25≦w≦1 μm,illustrating the amount of dispersion in the mode properties that can beexpected due to a varying film width. In all cases, the ss_(b) ⁰ modeevolves with decreasing film thickness into the TEM wave supported bythe background, but this evolution occurs more rapidly for a narrowerwidth. For a film of thickness t=20 nm, for example, from FIG. 11(a),the normalized phase constant of the mode supported by a film of widthw=1 μm is about 2.05, while that of the mode supported by a film ofwidth w=0.25 μm is already about 2. This fact is also supported by theresults plotted in FIG. 11(b) since the attenuation constant of the modeat a thickness of t=20 nm is closer to zero (the attenuation constant ofthe background) for narrow film widths compared to wider ones. Indeed,at a thickness of 10 nm, the attenuation of the mode for a width ofw=0.25 μm is more than an order of magnitude less than its attenuationat a width of w=1 μm (and more than an order of magnitude less than thatof the s_(b) mode supported by a metal film slab waveguide), indicatingthat this mode can be made even more long-ranging by reducing both thefilm thickness and its width.

[0156] The dispersion of the mode with increasing film thickness alsochanges as a function of film width, as seen from FIG. 11(a). This isdue to the fact that the amount of coupling between corners along thetop and bottom edges increases as the film narrows, implying that themode does not evolve with increasing thickness towards aplasmon-polariton mode supported by an isolated corner, but rathertowards a plasmon-polariton mode supported by the pair of cornerscoupled via these edges.

[0157] FIGS. 12(a) to 12(d) show contour plots of Re{S_(z)} related tothe ss_(b) ⁰ mode supported by films of thickness t=20 nm and variouswidths. The power confinement factor is also given for all cases, withthe area of the waveguide core A_(c) taken as the area of the metalregion. FIGS. 12(a) to 12(d) clearly illustrate how the fields becomeless confined to the lossy metal as its width decreases, explaining thereduction in attenuation shown in FIG. 11(b) at this thickness. Inaddition, the confinement factor ranges from 1.64% to 0.707% for thewidths considered, further corroborating this fact. The fields are alsoseen to spread out farther, not only along the horizontal dimension butalong the vertical one as well, as the film narrows. This indicates thatthe mode supported by a narrow film is farther along in its evolutioninto the TEM mode supported by the background, compared to a wider filmof the same thickness. It is also clear from FIGS. 12(a) to 12(d) thatthe trade-off between mode confinement and attenuation must be made byconsidering not only the film thickness but its width as well.

[0158] V. Effects Caused by Varying the Background Permittivity:Symmetric Structures

[0159] In this section, the changes in the propagation characteristicsof the ss_(b) ⁰ mode due to variations in the background permittivity ofthe waveguide. Only the ss_(b) ⁰ mode is considered since the maineffects are in general applicable to all modes. In order to isolate theeffects caused by varying the background permittivity, the width of themetal film was fixed to w=0.5 μm and its permittivity as well as theoptical free-space wavelength of analysis were set to the values used inthe previous sections. The relative permittivity of the backgroundε_(r,1) was taken as the variable parameter.

[0160] The dispersion with thickness of the ss_(b) ⁰ mode is shown inFIG. 13 for some background permittivities in the range 1≦ε_(r,1)≦4.FIGS. 14(a) to 14(d) compare contour plots of Re{S_(z)} related to thismode for a film of thickness t=20 nm and for the same set of backgroundpermittivities used to generate the curves plotted in FIG. 13. FromFIGS. 14(a) to 14(d); it is observed that reducing the value of thebackground permittivity causes a reduction in field confinement to themetal. This reduction in field confinement within the lossy metal inturn causes a reduction in the attenuation of the mode that can be quitesignificant, FIG. 13 showing a reduction of almost four orders ofmagnitude at a film thickness of t=20 nm, as the background relativepermittivity ranges from ε_(r,1)=4 to 1. It is also noted that the modeexhibits less dispersion with thickness as the background relativepermittivity is reduced, since the normalized phase constant curvesshown in FIG. 13 flatten out with a reduction in the value of thisparameter.

[0161] From FIGS. 14(a) to 14(d), it is seen that the mode power isconfined to within approximately one free-space wavelength in alldirections away from the film in all cases except that shown in FIG.14(d), where fields are significant up to about two free-spacewavelengths. In FIG. 14(c), the background permittivity is roughly thatof glass and from FIG. 13 the corresponding normalized attenuationconstant of the mode is about α/β₀=6.0×10⁻⁵. The associated mode powerattenuation in dB/mm, computed using the following formula:$\begin{matrix}{{Att} = {\alpha \times \frac{20}{1000}{\log_{10}(e)}}} & (16)\end{matrix}$

[0162] is about 5 dB/mm. This value of attenuation is low enough andfield confinement is high enough as shown in FIG. 14(c), to render thisparticular structure practical at this free-space wavelength forapplications requiring short propagation lengths.

[0163] The changes in mode properties caused by varying the backgroundpermittivity as discussed above are consistent with the changes observedfor the modes supported by a metal film slab waveguide and theobservations are in general applicable to the other modes supported by ametal film of finite width. In the case of the higher order modes (m>0)and those exhibiting a cutoff thickness (the as_(b) ^(m) modes for all mand the ss_(b) ^(m) modes for m>0) additional changes in the modeproperties occur. In particular, as the background permittivity isreduced, the cut-off widths of the higher order modes increase as do allrelevant cut-off thicknesses.

[0164] VI. Frequency Dependency of the ss_(b) ⁰ Mode Solutions:Symmetric Structures

[0165] In order to isolate the frequency dependency of the ss_(b) ⁰ modesolutions, the geometry of the metal film was held constant and thebackground relative permittivity was set to ε_(r,1)=4. The relativepermittivity of the metal film ε_(r,2) was assumed to vary with thefrequency of excitation according to Equation (1). In order to remainconsistent with [6], the values ω_(p)=1.29×10¹⁶ rad/s and1/ν=τ=1.25×10⁻¹⁴ s were adopted, though the latter do not generateexactly ε_(r,2)=−19−j0.53 at λ₀=0.633 μm, which is the value used in theprevious sections. This is due to the fact that values of ω_(p) and τare often deduced by fitting Equation (1) to measurements. The valuesused, however, are in good agreement with recent measurements made forsilver [3] and are expected to generate frequency dependent results thatare realistic and experimentally verifiable.

[0166] The dispersion characteristics of the ss_(b) ⁰ mode supported byfilms of width w=0.5 μm and w=1 μm, and thicknesses in the range 10≦t≦50nm are shown in FIGS. 15(a) and 15(b) for frequencies covering thefree-space wavelength range 0.5≦λ₀23 2 μm. Curves for the s_(b) modesupported by metal film slab waveguides (w=∞) of the same thicknessesare also shown for comparison.

[0167] The results given in FIG. 15(a) show that, in all cases, thenormalized phase constant of the modes tends asymptotically towards thatof the TEM wave supported by the background as the wavelength increases,and that the convergence to this value is steeper as the width of thefilm decreases (for a given thickness). The curves remain essentiallyunchanged in character as the thickness changes, but they shift upwardstoward the top left of the graph with increasing thickness, as shown.Convergence to the asymptote value with increasing wavelength suggeststhat the ss_(b) ⁰ mode evolves into the TEM mode supported by thebackground. It is noteworthy that the ss_(b) ⁰ mode can exhibit verylittle dispersion over a wide bandwidth, depending on the thickness andwidth of the film, though flat dispersion is also associated with lowfield confinement to the metal film.

[0168] The results plotted in FIG. 15(b) show in all cases a decreasingattenuation with increasing wavelength and the curves show a sharperdrop for a narrow film (w=0.5 μm) compared to a wide one (w=∞). Theattenuation curves look essentially the same for all of the filmthicknesses considered, though the range of attenuation values shiftsdownwards on the graph with decreasing film thickness.

[0169] FIGS. 16(a) to 16(f) give contour plots of Re{S_(z)} related tothe ss_(b) ⁰ mode for films of thickness t=20 nm and widths w=0.5 μm andw=1 μm, for three free-space wavelengths of operation: λ₀=0.6, 0.8 and1.21 μm. Comparing the contours shown in FIGS. 16(a) to 16(f), explainsin part the frequency dependent behaviour plotted in FIGS. 15(a) and15(b). FIGS. 16(a) to 16(f) show that the mode power contours spread outfarther from the film as the wavelength increases, which means that themode confinement to the metal region decreases, explaining in part thedecrease in losses and the evolution of the mode towards the TEM mode ofthe background, as shown in FIGS. 15(a) and 15(b). This behaviour ismore pronounced for the waveguide of width w=0.5 μm compared to thewider one of width w=1.0 μm.

[0170] There are two mechanisms causing changes in the ss_(b) ⁰ mode asthe frequency of operation varies. The first is geometrical dispersion,which changes the optical or apparent size of the film, and the secondis material dispersion, which is modelled for the metal region usingEquation (1). If no material dispersion is present, then the geometricaldispersion renders the film optically smaller as the free-spacewavelength is increased (an effect similar to reducing t and w) so, inthe case of the ss_(b) ⁰ mode, confinement to the film is reduced andthe mode spreads out in all directions away from the latter. Now basedon Equation (1), it is clear that the magnitude of the real part of thefilm's permittivity |Re{ε_(r,2)}| varies approximately in a 1/ω² or λ₀ ²fashion while the magnitude of its imaginary part |Im{ε_(r,2)}| variesapproximately in a 1/ω³ or λ₀ ³ fashion. However, an increase in|Re{ε_(r,2)}| reduces the penetration depth of the mode fields into themetal region and, combined with the geometrical dispersion, causes a netdecrease in mode attenuation with increasing wavelength, even though thelosses in the film increase in a λ₀ ³ fashion.

[0171]FIG. 15(b) shows that mode power attenuation values in the range10 to 0.1 dB/cm are possible near communications wavelengths (λ₀ ^(˜)1.5μm) using structures of reasonable dimensions: w^(˜)1.0 μm and t^(˜)15nm. Such values of attenuation are low enough to consider the ss_(b) ⁰mode as being long-ranging, suggesting that these waveguides arepractical for applications requiring propagation over short distances.As shown in the previous section, even lower attenuation values arepossible if the background permittivity is lowered. From FIGS. 16(e) and16(f), (case λ₀1.2 μm, which is near communications wavelengths), it isapparent that the mode power confinement is within one free-spacewavelength of the film, which should be tight enough to keep the modebound to the structure if a reasonable quality metal film of the rightgeometry can be constructed.

[0172] VII. Mode Characteristics and Evolution With Film Thickness:Small Asymmetry

[0173] A. Mode Solutions for a Metal Film Slab Waveguide

[0174] Effects on waveguiding characteristics of using an asymmetricwaveguide structure will now be discussed, beginning with thereproduction of results for an infinitely wide asymmetric metal filmwaveguide (similar to that shown in FIG. 17(a) but with w=∞), taken fromthe standard work on such structures [6]. In order to remain consistentwith their results, the optical free-space wavelength of excitation isset to λ₀=0.633 μm and the value they used for the relative permittivityof the silver film at this wavelength is used here: ε_(r,2)=19−j0.53.The relative permittivities of the bottom and top dielectric regions areset to ε_(r,1)=4 (n₁=2) and ε_(r,3)=3.61 (n₃=1.9); these values create astructure having a small asymmetry with respect to the horizontaldimension.

[0175] The dispersion curves of the s_(b) and a_(b) modes supported bythe infinitely wide structure were computed using the MoL and theresults are shown in FIGS. 18(a) and 18(b). From these figures, it isseen that the propagation constant of the a_(b) mode tends towards thatof the plasmon-polariton mode supported by the bottom interface, givenby Equations (14) and (15), as the thickness of the film increases. Itis also noted that this mode does not exhibit a cutoff thickness, whileit is clear that the s_(b) mode has one near t=18 nm. The propagationconstant of the s_(b) mode is seen to tend towards the value of aplasmon-polariton mode supported by the top interface as the thicknessincreases. These results are in perfect agreement with those reported in[6].

[0176] B. Modes Supported by a Metal Film of Width w=1 μm

[0177] The study proceeds with the analysis of the structure shown inFIG. 17(a) for the case w=1 μm. The material parameters and free-spacewavelength that were used in the previous case w=∞ were also used here.The dispersion curves for the first seven modes were computed using theMoL and the results are shown in FIGS. 18(a) and 18(b).

[0178] In this asymmetric structure, true field symmetry exists onlywith respect to the y axis. With respect to the horizontal dimension,the modes have a symmetric-like or asymmetric-like field distributionwith field localization along either the bottom or top metal-dielectricinterface. The modes that have a symmetric-like distribution withrespect to the horizontal dimension are localized along themetal-dielectric interface with the lowest dielectric constant, whilemodes that have an asymmetric-like distribution with respect to thisaxis are localized along the metal-dielectric interface with the highestdielectric constant. This behaviour is consistent with that observed forasymmetric metal slab waveguides.

[0179] The mode nomenclature adopted for symmetric structures can beused without ambiguity to describe the modes supported by asymmetricstructures as long as the modes are identified when the metal film isfairly thick, before significant coupling begins to occur through themetal film, and while the origin of the mode can be identifiedunambiguously. As the metal film thickness decreases, the modes (andtheir fields) can evolve and change considerably more in an asymmetricstructure compared to a symmetric one. The number of extrema in the maintransverse electric field component of the mode is counted along thelateral dimension at the interface where the fields are localized. Thisnumber is then used in the mode nomenclature.

[0180] It was observed in Section III that the modes supported bysymmetric structures are in fact supermodes created from a coupling of“edge” and “corner” modes supported by each metal-dielectric interfacedefining the structure. As the thickness and width of the metaldecrease, the coupling between these interface modes intensifies leadingto dispersion and possibly evolution of the supermode. In asymmetricstructures, the bound modes are also supermodes created in a similarmanner, except that dissimilar interface modes may couple to each otherto create the supermode. For instance, a mode having one field extremumalong the top interface (along the top edge bounded by the corners) maycouple with a mode having three extrema along the bottom interface. Themain selection criterion determining which interface modes will coupleto create the supermode is similarity in the value of their propagationconstants. For all modes supported by an asymmetric structure, anapparent symmetry or asymmetry with respect to the horizontal dimensioncan still be observed in the corner modes.

[0181] The sa_(b) ⁰, aa_(b) ⁰, ss_(b) ⁰ and as_(b) ⁰ modes are thefundamental modes supported by the structure. The sa_(b) ⁰ and aa_(b) ⁰modes are comprised of coupled corner modes, resembling thecorresponding modes in a symmetric structure, except that the fields arelocalized near the substrate. These two modes do not change in characteras the thickness of the film decreases. A narrowing of the metal filmwould eventually break the degeneracy observed in FIGS. 18(a) and 18(b).

[0182] For a sufficiently large thickness (about 100 nm for the presentstructure), the ss_(b) ⁰ and as_(b) ⁰ modes are comprised of coupledcorner modes much like the corresponding modes in a symmetric structureexcept that the fields are localized near the superstrate. As thethickness of the metal film decreases, both of these modes begin toevolve, changing completely in character for very thin films. FIGS.19(a) to 19(d) show the evolution of the E_(y) field component relatedto the ss_(b) ⁰ mode as the thickness of the film ranges from 100 nm(FIG. 19(a)) to 40 nm (FIG. 19(d)). It is clearly seen that the modeevolves from a symmetric-like mode having fields localized near thesuperstrate to an asymmetric-like mode having fields localized along thesubstrate-metal interface. A similar evolution is observed for theas_(b) ⁰ mode. This change in character is also apparent in theirdispersion curves: they follow the general behaviour of a symmetric-likemode for large thicknesses but then slowly change to follow thebehaviour of an asymmetric-like mode as the thickness decreases. Sincethe substrate dielectric constant is larger than the superstratedielectric constant, the mode is “pulled” from a symmetric-like mode toan asymmetric-like mode (having field localization at thesubstrate-metal interface) as the metal film becomes thinner.

[0183] FIGS. 20(a) to 20(d) show the E_(y) field component related tothe ss_(b) ¹ and sa_(b) ¹ modes for two film thicknesses. From theseFigures it is noted that the top and bottom edge modes comprising asupermode are different from each other. In FIG. 20(a), for instance, itis seen that the bottom edge mode has three extrema and is of higherorder than the top edge mode which has one extremum. A similarobservation holds for FIG. 20(c), where it can be seen that the bottomedge mode has one extremum while the top one has none. In thisstructure, the substrate has a higher dielectric constant than thesuperstrate so the phase constant of a particular substrate-metalinterface mode will be higher than the phase constant of the same modeat the metal-superstrate interface. Since a supermode is created from acoupling of edge modes having similar propagation constants, it shouldbe expected that, in an asymmetric structure, different edge modes maycouple to create a supermode. In general, higher-order modes havesmaller values of phase constant compared to lower-order modes, so instructures having ε₃<ε₁, all supermodes are comprised of a bottom edgemode of the same order or higher than the top edge mode, as shown inFIGS. 20(a) to 20(d). If ε₃>ε₁, then the opposite statement is true.

[0184] A careful inspection of the fields associated with the ss_(b) ¹,sa_(b) ¹ and aa_(b) ² modes reveals that, as the thickness of the filmdecreases, the mode fields may evolve in a smooth manner similar to thatshown in FIGS. 19(a) to 19(d), but, in addition, a change or “switch” ofthe constituent edge modes may also occur. For instance, from FIG.20(c), the sa_(b) ¹ mode is seen to comprise a substrate-metal interfacemode having one extremum for a film thickness of 100 nm, while for athickness of 60 nm the substrate-metal interface mode has three extrema,as shown in FIG. 20(d). Since higher-order modes have in general lowerphase constants than lower-order modes, this change in edge modes causesa reduction in the phase constant of the sa_(b) ¹ mode in theneighbourhood of 60 nm, as shown in FIG. 18(a). Another change occursnear 40 nm as the corner modes switch from being symmetric-like (as inFIGS. 20(c) and 20(d)) to being asymmetric-like with respect to thehorizontal dimension. This change is again reflected in the dispersioncurve of the sa_(b) ¹ mode as its phase constant is seen to increasewith a further decrease in thickness. In general, the changes in theedge and corner modes are consistent with the directions taken by thedispersion curves as the film thickness decreases, thus explaining theoscillations in the curves seen in FIGS. 18(a) and 18(b).

[0185] The only potentially long-ranging mode supported by thisstructure at the wavelength of analysis is the ss_(b) ¹ mode. As shownin FIGS. 18(a) and 18(b), the mode has a cutoff thickness near t=22 nmand although the attenuation drops quickly near this thickness, itshould be remembered that the field confinement does so as well.Furthermore, the spatial distribution of the main transverse fieldcomponent related to this mode evolves with decreasing thickness in themanner shown in FIGS. 20(a) and 20(b), such that near cutoff the spatialdistribution has strong extrema along the top and bottom edges. Theseextrema render the mode less excitable using an end-fire technique, socoupling losses would be higher compared to the fundamental symmetricmode in symmetric waveguides. Also, the fact that the mode would beoperated near its cutoff thickness implies that very tight tolerancesare required in the fabrication of structures. Nevertheless, it shouldbe possible to observe propagation of this mode in a suitable structureusing an end-fire experiment.

[0186] VIII. Mode Characteristics and Evolution With Film Thickness:Large Asymmetry

[0187] A. Mode Solutions for a Metal Film Slab Waveguide

[0188] The study proceeds with the analysis of structures having a largedifference in the dielectric constants of the substrate and superstrate.With respect to FIG. 17(a), the relative permittivities of the substrateand superstrate are set to ε_(r,1)=4 (n₁=2) and ε_(r,3)=2.25 (n₃=1.5),respectively, the width of the metal film is set to w=∞, and thedielectric constant of the metal region and the wavelength of analysisare set to the same values as in Section III. The dispersion curves ofthe s_(b) and a_(b) modes supported by this structure can be seen inFIGS. 21(a) and 21(b). Comparing with FIGS. 18(a) and 18(b), it isobserved that the s_(b) mode has a larger cutoff thickness in astructure having a large asymmetry than in a structure having similarsubstrate and superstrate dielectric constants. The results shown werecomputed using the MoL and are in perfect agreement with those reportedin [6].

[0189] B. Modes Supported by a Metal Film of Width w=1 μm

[0190] The structure shown in FIG. 17(a) was analyzed using the MoL forw=1 μm and for the same material parameters and free-space wavelength asthose given above for w=∞. The dispersion curves of the first six modessupported by the structure are shown in FIGS. 21(a) and 21(b).

[0191] An inspection of the mode fields related to the sa_(b) ⁰ andaa_(b) ⁰ modes reveals that these modes are again comprised of coupledcorner modes with fields localized at the substrate-metal interface. Themodes do not change in character as the thickness of the film decreasesand a narrowing of the metal film would eventually break the degeneracyobserved in FIGS. 21(a) and 21(b).

[0192] The spatial distribution of the E_(y) field component related tothe, ss_(b) ⁰, as_(b) ⁰, sa_(b) ¹ and aa_(b) ² modes is given in FIGS.22(a) to 22(d). It is noted from this figure that in all cases themetal-superstrate interface modes are similar: they have fields with noextrema along the interface but rather that are localized near thecorners and have either a symmetric or asymmetric distribution withrespect to the y axis. These corner modes are in fact the lowest ordermodes supported by the metal-superstrate interface; they have thelargest value of phase constant and thus are most likely to couple withedge modes supported by the substrate-metal interface to form asupermode. From FIGS. 22(a) and 22(b) it is observed that thesubstrate-metal interface modes comprising the ss_(b) ⁰ and as_(b) ⁰modes are of very high order. This is expected since the substratedielectric constant is significantly higher than the superstratedielectric constant and higher order modes have lower values of phaseconstant.

[0193] The ss_(b) ⁰ and as_(b) ⁰ modes shown in FIGS. 22(a) and 22(b)indeed have fields that are localized along the metal-superstrateinterface, while the sa_(b) ¹ and aa_(b) ² modes shown in FIGS. 22(c)and 22(d) have fields that are localized along the substrate-metalinterface.

[0194] One effect, caused by increasing the difference between thesubstrate and superstrate dielectric constants, is that the differencebetween the orders of the top and bottom edge modes comprising asupermode can increase. This effect can be observed by comparing FIG.19(a) with FIG. 22(a). In the former, there is no difference between theorders of the top and bottom edge modes, while in the latter thedifference in the orders is 5. Another effect is that the degree offield localization increases near the interface between the metal andthe dielectric of higher permittivity, for all modes that areasymmetric-like with respect to the horizontal dimension. This effectcan be seen by comparing the fields related to the sa_(b) ¹ mode shownin FIGS. 22(c) and 20(c). A comparison of the fields related to thesa_(b) ⁰ and aa_(b) ⁰ modes reveals that this effect is present in thesemodes as well.

[0195] From the dispersion curves shown in FIG. 21(a), it is apparentthat the normalized phase constant of all modes converge with increasingfilm thickness to normalized phase constants in the neighbourhood ofthose supported by plasmon-polariton waves localized along theassociated isolated edge. The normalized phase constants of modes havingfields localized at the substrate-metal interface, converge withincreasing film thickness to normalized phase constants in theneighbourhood of that related to the a_(b) mode, while the normalizedphase constants of modes having fields localized along themetal-superstrate interface converge to values near that of the s_(b)mode. This behaviour is present though less apparent, in structureswhere the asymmetry is smaller, such as the one analyzed in Section VII.

[0196] Comparing FIGS. 18(a) and 18(b) with FIGS. 21(a) and 21(b), it isnoted that the dispersion curves of the modes are much smoother when thedifference in the substrate and superstrate dielectric constants islarge. This is due to the fact that the edge modes comprising thesupermodes are less likely to change or switch as they do in a structurehaving similar substrate and superstrate dielectric constants. Thusmodes that start out being symmetric-like with respect to the horizontaldimension remain so as the thickness of the film decreases. The cutoffthickness of the symmetric-like modes also increases as the differencebetween the substrate and superstrate dielectric constants increases.

[0197] It is apparent that introducing a large asymmetry can hamper theability of the structure to support useful long-ranging modes. Any modethat is long-ranging would likely have fields with numerous extremaalong the width of the interface between the metal film and thedielectric of higher permittivity, as shown in FIGS. 22(a) and 22(b).

[0198] IX. Mode Dispersion with Film Width: Small Asymmetry

[0199] An asymmetric structure comprising the same dielectrics as thestructures studied in Section VII, but having a metal film of widthw=0.5 μm, was analyzed at the same free-space wavelength in order todetermine the impact of a narrowing film width on the modes supported.The structure was analyzed using the MoL and FIGS. 23(a) and 23(b) givethe dispersion curves obtained for the first few modes supported.

[0200] Comparing FIGS. 23(a) and 23(b) with FIGS. 18(a) and 18(b)reveals that reducing the width of the film does not cause major changesin the behaviour of the fundamental modes, but does have a major impacton the higher order modes. It is noted that reducing the film widthincreases the cutoff thickness of the ss_(b) ¹ mode. This higher ordermode is symmetric-like with respect to the horizontal dimension, and thecutoff thickness of the symmetric-like modes in general increases as thewidth of the film decreases due to a reduction in field confinement tothe metal film.

[0201] The aa_(b) ² mode was sought but not found for this film width.

[0202] It is also noted by comparing FIGS. 23(a) and 23(b) with FIGS.18(a) and 18(b) that the sa_(b) ¹ mode evolves quite differentlydepending on the width of the film. For a film width of w=1 μm, the modefollows the general behaviour of an asymmetric-like mode whereas, for afilm width of w=0.5 μm, the mode evolves as a symmetric-like mode, andhas a cutoff thickness near t=27 nm. When the film is wide, it becomespossible for numerous higher order edge modes (having similar values ofphase constant) to be supported by the substrate-metal ormetal-superstrate interfaces, so edge modes comprising a supermode arelikely to change or switch as the thickness of the film is reduced, asshown in FIGS. 20(c) and 20(d). For a narrow metal film, some of thehigher order edge modes may be cutoff, thus rendering changes in edgemodes impossible. In such a case, the supermode-may be forced to evolvein a smooth manner with decreasing film thickness. A close inspection ofthe mode fields related to the sa_(b) ¹ mode for a film width of w=0.5μm reveals that there are no changes to the edge modes as the thicknessdecreases; rather the mode evolves smoothly from its field distributionat a large thickness (similar to that shown in FIG. 20(c)) to having asymmetric-like distribution with only one extremum along the top andbottom edges of the film. A change in behaviour due to a change in thewidth of the metal film was observed only for the sa_(b) ¹ mode in thisstudy, but such changes are in general not limited to this mode.

[0203] The sa_(b) ¹ and ss_(b) ¹ modes could be made to propagate overuseful distances in this structure, if they are excited near theircutoff thicknesses. However, the difficulties outlined in Section VII Bregarding the excitation of modes near cutoff also hold here.

[0204] X. Evolution of the ss_(b) ⁰ and sa_(b) ¹ Modes With StructureAsymmetry

[0205] The ss_(b) ⁰ and sa_(b) ¹ modes are of practical interest. Thess_(b) ⁰ mode is the main long-ranging mode supported by symmetricfinite-width metal film structures, and, as demonstrated in the previoussection, the sa_(b) ¹ mode can be the main long-ranging mode supportedby asymmetric finite-width structures. In metal films of the rightthickness, they are also the modes that are the most suitable toexcitation in an end-fire arrangement.

[0206] Structures comprising a substrate dielectric having n₁=2, of ametal film of width w=0.5 μm, and of various superstrate dielectricshaving n₃=2, 1.99, 1.95 and 1.9 were analyzed at the same free-spacewavelength as in Section VII. The equivalent permittivity of the metalfilm was also set to the same value as in Section VII. These analyseswere performed in order to investigate the effects on the propagationcharacteristics of the ss_(b) ⁰ and sa_(b) ¹ modes caused by a slightdecrease in the superstrate permittivity relative to the substratepermittivity. FIGS. 24(a) and 24(b) show the dispersion curves with filmthickness, obtained for these modes in the four structures of interest.

[0207] As seen in FIG. 24(a) and its inset, the dispersion curves of themodes intersect at a certain film thickness only for the symmetric case(n₃=n₁). As soon as some degree of asymmetry exists, the curves nolonger intersect, though they may come quite close to each other if theasymmetry is small, as seen in the case of n₃=1.99. As the degree ofasymmetry increases, the separation between the curves increases.

[0208] The evolution with film thickness of the sa_(b) ¹ mode is shownin FIGS. 25(a) to 25(d) for the case n₃=1.99 and for thicknesses aboutt=59 nm (near the maximum in its phase dispersion curve). The evolutionof this mode for the cases n₃=1.95 and 1.9 is similar to that shown. Theevolution with film thickness of the ss_(b) ⁰ mode is similar in, thesestructures to the evolution shown in FIGS. 19(a) to 19(d) for the casew=1 μm and n₃=1.9. Comparing FIGS. 25(a) to 25(d) and FIGS. 19(a) to19(d), reveals that the modes “swap” character near t=59 nm. For filmthicknesses sufficiently above this value, the modes exhibit theirdefining character as shown in FIGS. 19(a) and 25(a), but for filmthicknesses below it, each mode exhibits the other's character, as shownin FIGS. 19(d) and 25(d). This character swap is present for the threecases of asymmetry considered here (n₃=1.99, 1.95 and 1.9) and explainsthe behaviour of the dispersion curves shown in FIGS. 24(a) and 24(b).

[0209] From FIGS. 24(a) and 24(b), it is noted that a cutoff thicknessexists for the long-ranging mode as soon as an asymmetry is present inthe structure. It is also observed that the cutoff thickness increaseswith increasing asymmetry. In the case of n₃=1.99, the cutoff thicknessof the mode is near t=12 nm, while for n₃=1.9 the cutoff thickness isnear t=27 nm. As the width of the metal film w increases, the cutoffthickness of the sa_(b) ¹ mode decreases as long as the mode remainslong-ranging (recall that the character of this mode may also changewith film width such that its behaviour is similar to the a_(b) mode inthe corresponding slab structure, as shown in FIGS. 18(a) and 18(b)).Also, it is clear from FIGS. 23(a) and 23(b) that the cutoff thicknessof the sa_(b) ¹ mode is greater than the cutoff thickness of the s_(b)mode supported by the corresponding slab structure. These results implythat the long-ranging mode supported by a thin narrow metal film is moresensitive to differences in the superstrate and substrate permittivitiesthan the s_(b) mode supported by the corresponding slab structure. Thisis reasonable in light of the fact that, in finite-width structures, themode fields tunnel through the metal as in slab structures, but, inaddition, the fields also wrap around the metal film.

[0210]FIG. 24(b) shows that near cutoff, the attenuation of the sa_(b) ¹mode supported by an asymmetric structure drops much more rapidly thanthe attenuation of the ss_(b) ⁰ mode supported by a symmetric structure.Thus, a means for range extension, similar to that observed inasymmetric slab structures [7], exists for metal films of finite width,though the difficulties related to the excitation of a mode near itscutoff thickness, as described in Section VII B, also apply here.

[0211] FIGS. 26(a) to 26(d) show contour plots of

{S_(z)} associated with the long-ranging modes for the four cases ofsuperstrate permittivity considered. S_(z) is the z-directed componentof the Poynting vector and its spatial distribution is computed from thespatial distribution of the mode fields using:

S _(z)=(E _(x) H _(y) ^(*) −E _(y) H _(x) ^(*))/2   (6)

[0212] where H_(x,y) ^(*) denotes the complex conjugate of H_(x,y). FIG.26(a) shows the contour plot associated with the ss_(b) ⁰ mode supportedby a symmetric structure (n₃=n₁=2) of thickness t=20 nm. FIGS. 26(b),(c)and (d) show contours associated with the sa_(b) ¹ mode for the threecases of structure asymmetry considered. The contour plots shown inFIGS. 26(b),(c) and (d) are computed for film thicknesses slightly abovecutoff, representative of the thicknesses that would be used to observethese long-ranging modes experimentally. From those figures, it is notedthat the contour plots become increasingly distorted and the fieldsincreasingly localized at the metal-superstrate interface as the degreeof asymmetry in the structure increases. It is also apparent bycomparing FIGS. 26(a) and 26(d), that in an end-fire experiment, lesspower should be coupled into the sa_(b) ¹ mode supported by theasymmetric structure with n₃=1.9, compared to the ss_(b) ⁰ modesupported by the symmetric structure. End-fire coupling losses seem toincrease with increasing structure asymmetry.

[0213] The high sensitivity of the long-ranging mode supported by thinmetal films of finite width, to structure asymmetry, is potentiallyuseful. A small induced asymmetry (created via an electro-optic effectpresent in the dielectrics say) can evidently effect a large change inthe propagation characteristics of the long-ranging mode. From FIGS.24(a) and 24(b), it is apparent that a difference between the substrateand superstrate refractive indices as small as n₁-n₃=Δn=0.01 issufficient to create an asymmetric structure where the long-ranging modehas a cutoff thickness of about t=12 nm. From FIG. 24(a), a slightlylarger difference of Δn=0.05 changes the normalized phase constant ofthe long-ranging mode by Δ(β/β₀)≈0.025 for a metal film thickness oft=20 nm. Both of these effects are potentially useful.

[0214] Asymmetric structures having superstrate dielectric constantsthat are slightly greater than that of the substrate were also analyzed.The substrate dielectric constant was set to n₁₌2 and superstratedielectrics having n₃=2.01, 2.05 and 2.1 were considered for the samemetal, film width and operating wavelength. The results are similar tothose presented in FIGS. 24 through 26 and the cut-off thicknesses arenear those shown in FIG. 24(b). Though the results are not identical,there is no major difference between the behaviour of the ss_(b) ⁰ andsa_(b) ¹ modes whether ε₁>ε₂ or ε₁<ε₃ as long as the permittivities aresimilar.

[0215] XI. Conclusion

[0216] The purely bound optical modes supported by thin lossy metalfilms of finite width, embedded in an “infinite” homogeneous dielectrichave been characterized and described. The modes supported by thesesymmetric structures are divided into four families depending on thesymmetry of their mode fields and none of the modes are TM in nature (asthey are in the metal film slab waveguide). In addition to the fourfundamental modes that exist, numerous higher order modes are supportedas well. A proposed mode nomenclature suitable for identifying them hasbeen discussed. The dispersion of the modes with film thickness has beenassessed and the behaviour in general terms found to be consistent withthat of the purely bound modes supported by the metal film slabwaveguide. In addition, it has been found that one of the fundamentalmodes and some higher order modes have cut-off thicknesses. Modedispersion with film width has also been investigated and it has beendetermined that the higher order modes have a cut-off width, below whichthey are no longer propagated. The effect of varying the backgroundpermittivity on the modes has been investigated as well, and the generalbehaviour found to be consistent with that of the modes supported by ametal film slab waveguide. In addition it was determined that thecut-off width of the higher order modes decreases with decreasingbackground permittivity and that all cut-off thicknesses are increased.

[0217] One of the fundamental modes supported by the symmetricstructures, the ss_(b) ⁰ mode, exhibits very interesting characteristicsand is potentially quite useful. This mode evolves with decreasing filmthickness towards the TEM wave supported by the background, (anevolution similar to that exhibited by the s_(b) mode in metal film slabwaveguides), its losses and phase constant tending asymptoticallytowards those of the TEM wave. In addition, it has been found thatdecreasing the film width can reduce the losses well below those of thes_(b) mode supported by the corresponding metal film slab waveguide.Reducing the background permittivity further reduces the losses.However, a reduction in losses is always accompanied by a reduction infield confinement to the waveguide core, which means that both of theseparameters must be traded-off one against the other. Furthermore,carefully selecting the film's thickness and width can make the ss_(b) ⁰mode the only long-ranging mode supported. It has also been demonstratedthat mode power attenuation values in the range of 10 to 0.1 db/cm areachievable at optical communications wavelengths, with even lower valuespossible. Finally, evolved into its most useful form, the ss_(b) ⁰ modehas a field distribution that renders it excitable using end-firetechniques.

[0218] The existence of the ss_(b) ⁰ mode in a symmetric structure, aswell as its interesting characteristics, makes the finite-width metalfilm waveguide attractive for applications requiring short propagationdistances. The waveguide offers two-dimensional field confinement in thetransverse plane, rendering it useful as the basis of an integratedoptics technology. Interconnects, power splitters, power couplers andinterferometers could be built using the guides. Finally, the structuresare quite simple and so should be inexpensive to fabricate.

[0219] The long-ranging modes supported by asymmetric structures offinite width have a rapidly diminishing attenuation near their cutoffthickness (like asymmetric slab structures). The rate of decrease of theattenuation with decreasing thickness near cutoff is greater than therate related to the ss_(b) ⁰ mode in symmetric structures. However fieldconfinement also diminishes rapidly near cutoff, implying that thestructures ought to be fabricated to very tight tolerances and that allmetal-dielectric interfaces should be of the highest quality. It hasalso been found that decreasing the width of the film increases thecutoff thickness of the main long-ranging mode. Below this cutoffthickness, no purely bound long-ranging mode exists. The long-rangingmodes supported by metal films of finite-width are thus more sensitiveto the asymmetry in the structure as compared to the s_(b) modesupported by similar slab waveguides. This is a potentially usefulresult in that a small induced change in substrate or superstraterefractive index can have a greater impact on the long-ranging modesupported by a finite-width structure as compared to a similar slabwaveguide.

[0220] Parts of the foregoing theoretical discussion have been publishedby the inventor in references [13][20], [44] and [45].

SPECIFIC EMBODIMENTS AND EXAMPLES OF APPLICATION

[0221] Examples of practical waveguide structures, and integrated opticsdevices which can be implemented using the invention, will now bedescribed with reference also to FIGS. 27 to 42, 51 and 52. Unlessotherwise stated, where a waveguide structure is shown, it will have ageneral construction similar to that shown in FIGS. 1(a) and 1(b) orthat shown in FIGS. 17(a) and 17(b).

[0222] Waveguides, structures and devices disclosed herein operate withradiation:

[0223] having a wavelength such that a plasmon-polariton wave issupported;

[0224] at optical wavelengths;

[0225] at optical communications wavelengths;

[0226] at wavelengths in the range of 800 nm to 2 μm;

[0227] at wavelengths near 1550 nm;

[0228] at wavelengths near 1310 nm;

[0229] at wavelengths near 850 nm;

[0230] at wavelengths near 980 nm.

[0231] It should be appreciated that references to wavelength should beinterpreted as meaning the centre free space wavelength of the spectrumassociated with the input radiation.

[0232] The waveguide structure 100 shown in FIGS. 1(a) and 1(b)comprises a strip of finite thickness t and width w of a first materialhaving a high free (or almost free) charge carrier density, surroundedby a second material which has a very low free (or almost free) chargecarrier density. The strip material can be a metal or a highly dopedsemiconductor and the background material can be a dielectric, or anundoped or low doped semiconductor. The thickness t and the width w ofthe strip are selected such that the waveguide supports the mainlong-ranging plasmon-polariton mode of interest, the ss_(b) ⁰ mode, atthe free-space wavelength of interest.

[0233] The waveguides will propagate plasmon-polariton waves if thestrip has a width in the range from about 0.1 μm to about 12 μm and athickness in the range from about 5 nm to about 100 nm, particulardimensions depending on the index of refraction of the surroundingmaterial, the strip material and the free-space wavelength of operation.

[0234]FIGS. 51 and 52 give the mode power attenuation for examplewaveguides constructed from strips of gold (Au) and aluminium (Al),respectively, each embedded in silicon dioxide (SiO₂), for variouswidths and thicknesses of the metal strip. The analyses were carried outat an optical free space wavelength of 1550 nm. The curves show thatvery low attenuation values can be obtained with metal strips ofpractical dimensions. Generally, the attenuation using the gold strip isabout one half of that obtained with the aluminium strip having similardimensions. Suitable ranges for the dimensions of the strip, in theseparticular waveguide examples, can be deduced from FIGS. 51 and 52. Asuitable range for the thickness of the strip is from about 5 nm toabout 50 nm, and a suitable range for the width of the strip is fromabout 0.5 μm to about 12 μm. For either of these metals, particularlygood dimensions are a thickness of about 20 nm and a width of about 4μm; such a strip yields a low loss waveguide having a mode size of about10 μm.

[0235] For a waveguide structure having particular materials for thestrip and surrounding material, operable at a particular wavelength,there will be a range of thickness and width for the strip which willallow the plasmon-polariton wave to be supported. For metallic stripssurrounded by material such as silicon dioxide, silicon oxynitride orsilicon nitride, having a refractive index in the range from about 1.4to about 2.0, the range of dimensions for the metallic strip is thewidth in the range from about 0.5 μm to about 12 μm and the thickness inthe range from about 5 nm to about 50 nm. Such waveguide structuressupport effectively the propagation of a plasmon-polariton wave having awavelength in the range from about 0.8 μm to about 2 μm. For operationin the wavelength range from about 1.3 μm to about 1.7 μm, gooddimensions for the metallic strip are a width in the range from about0.7 μm to about 8 μm and a thickness in the range from about 15 nm toabout 25 nm. A particularly good choice for the dimensions of themetallic strip is a width of about 4 μm and a thickness of about 20 nmfor operation at a wavelength near 1.55 μm. Other materials having anindex of refraction approximately in the same range require stripdimensions in approximately the same ranges.

[0236] For strips surrounded by materials such as lithium niobate orPLZT, having a refractive index in the range from about 2 to about 2.5,and required to support effectively the propagation of aplasmon-polariton wave having a free space wavelength in the range fromabout 0.8 μm to about 2 μm, the ranges of dimensions for the strippreferably include a width in the range from about 0.15 μm to about 6 μmand a thickness in the range from about 5 nm to about 80 nm. Foroperation in the wavelength range from about 1.3 μm to about 1.7 μm,good dimensions for the strip are a width in the range from about 0.4 μmto about 2 μm and thickness in the range from about 15 nm to about 40nm. For operation at a wavelength near 1.55 μm, the dimensions of thestrip preferably are a width of about 1 μm and a thickness of about 20nm. Other surrounding materials having an index of refractionapproximately in the same range require strip dimensions inapproximately the same ranges.

[0237] For strips surrounded by materials such as silicon, galliumarsenide, indium phosphide or other material having a refractive indexin the range from about 2.5 to about 3.5, and required to supporteffectively the propagation of a plasmon-polariton wave having a freespace wavelength in the range from about 0.8 μm to about 2 μm, theranges of dimensions for the strip preferably include a width in therange from about 0.1 μm to about 2 μm and thickness in the range fromabout 5 nm to about 30 nm. For operation in the wavelength range fromabout 1.3 μm to about 1.7 μm, good dimensions for the strip are a widthin the range from about 0.13 μm to about 0.5 μm and thickness in therange from about 10 nm to about 20 nm. For operation at a wavelengthnear 1.55 μm, the dimensions of the strip preferably are a width ofabout 0.25 μm and thickness of about 15 nm. Other surrounding materialshaving an index of refraction approximately in the same range requirestrip dimensions in approximately the same ranges.

[0238] Unless otherwise stated, when waveguide structure dimensions arementioned from this point onward, they refer to the Au/SiO₂ materialcombination at an operating optical free-space wavelength of 1550 nm.

[0239] The plasmon-polariton wave which propagates along a waveguidestructure may be excited by an appropriate optical field incident at oneof the ends of the waveguide, as in an end-fire configuration, and/or bydifferent radiation coupling means. In order to achieve good couplingefficiency with the input and output means, the plasmon-polariton wavepreferably has a transverse electric field component that overlaps wellwith that of the fundamental mode of propagation supported by the inputand output means. Tapered input and output strip sections can be used tomatch the mode sizes of the plasmon-polariton waveguide and thewaveguide used as the input and output means, in order to achieve highcoupling efficiency. An example of an appropriate end-fire arrangementcomprises a standard optical fibre butt-coupled to the input of thewaveguide (the output of the waveguide can also be butt-coupled to afibre).

[0240] An example waveguide structure achieving high coupling efficiencywith standard optical fibre at a free space wavelength near 1.55 μmcomprises an Au strip surrounded by SiO₂, the strip having a thicknessof about 20 nm and a width in the range from about 4 μm to about 8 μm.Alternative means for exciting the waveguide include excitation at anintermediate position using, for example, a prism and the so-calledattenuated total reflection method (ATR).

[0241] Suitable materials for the surrounding material include (but arenot limited to) glass, glasses, quartz, polymer, silicon dioxide,silicon nitride, silicon oxynitride, and undoped or very lightly dopedGaAs, InP or Si. The material may comprise single crystal or partiallycrystalline material. The surrounding material may compriseelectro-optic crystals or material such as lithium niobate (LiNbO₃),PLZT or electro-optic polymers. The surrounding material is notnecessarily homogeneous.

[0242] Suitable materials for the strip include (but are not limited to)metals, semi-metals, and highly n- or p-doped semiconductors such asGaAs, InP or Si. Suitable metals for the strip may comprise, a singlemetal, or a combination of metals, selected from the group Au, Ag, Cu,Al, Pt, Pd, Ti, Ni, Mo, and Cr, preferred metals being Au, Ag, Cu, andAl. Metal silicides such as CoSi₂ are particularly suitable for use withSi as the surrounding material. A single material or combination ofmaterials which behave like metals, such as Indium Tin Oxide (ITO) canalso be used. In the context of this specification, the term “metallic”substances embraces such material(s). The metallic strip is notnecessarily homogeneous.

[0243] Particularly suitable combinations of materials include Au forthe strip and SiO₂ for the surrounding material, or Au for the strip andLiNbO3 for the surrounding material, or Al or CoSi₂ for the strip and Sifor the surrounding material, or Au for the strip and polymer for thesurrounding material, or gold for the strip and PLZT for the surroundingmaterial. These combinations are preferred because, in each case, thestrip material exhibits low optical absorption and is chemicallycompatible with the surrounding material. Devices may be fabricatedusing known deposition techniques or wafer bonding and polishing for thecladding materials, and known lithographic and deposition techniques forthe strip.

[0244] The waveguide structures may comprise: a strip that ishomogeneous and a surrounding material that is homogeneous, a strip thatis homogeneous and a surrounding material that is inhomogeneous; a stripthat is inhomogeneous and a surrounding material that is homogeneous; ora strip that is inhomogeneous and a surrounding material that isinhomogeneous. An inhomogeneous strip can be formed from a continuouslyvariable material composition, or strips or laminae. An inhomogeneoussurrounding material can be formed from a continuously variable materialcomposition, or strips or laminae.

[0245] Generally, a plasmon-polariton waveguide having a metallic stripof large aspect ratio supports substantially TM polarized light, ie.:the transverse electric field component of the plasmon-polariton wave,the ss_(b) ⁰ mode, is aligned substantially with the normal to thelargest dimension in the waveguide cross-section. Thus, a straightwaveguide 100 with the dimensions set out above is polarisationsensitive. The plasmon-polariton wave is highly linearly polarised inthe vertical direction, i.e. perpendicular to the plane of the strip.Hence, it may serve as a polarisation filter, whereby substantially onlya vertical polarised mode (aligned along the y-axis as defined in FIG.1(a)) of the incident light is guided.

[0246] The length l shown in FIG. 1(b) is arbitrary and will be selectedto implement a desired interconnection.

[0247]FIG. 27 shows a transition waveguide section 102 having steppedsides which can be used to interconnect two sections of waveguide havingdifferent widths. The larger width can be used to more effectivelycouple the waveguide to input/output means, such as fibres. The reducedwidth helps to reduce the insertion loss of the waveguide. Preferredwidths are about W₂=8 μm to couple to single mode fibre and about W₁=4μm for the waveguide width. Any symmetry of the structure shown can beused.

[0248]FIG. 28 shows an angled section 104 which can be used as aninterconnect. Its dimensions, W₁, W₂ and 1 and the angles φ₁ and φ₂, areadjusted for a particular interconnection application as needed. Usuallythe angles are kept small, in the range of about 1 to about 15 degreesand the input and output widths are usually similar, about 4 μm.Although the sides of the angled section 104 shown in FIG. 28 aretapered, they could be parallel. It should also be appreciated that theangle of the inclination could be reversed, i.e. the device could besymmetrical about the bottom right hand corner shown in FIG. 28 ortransposed about that axis if not symmetrical about it.

[0249]FIG. 29 shows a tapered waveguide section 106, which can be usedto interconnect two waveguides of different widths. The length of thetaper is usually adjusted such that the angles are small, preferably inthe range from about 0.5 to about 15 degrees. The taper angles at thetwo sides are not necessarily the same. Such a configuration might beused as an input port, perhaps as an alternative to the layout shown inFIG. 27, or as part of another device, such as a power splitter. Thelarger width can be used to more effectively couple the waveguide toinput/output means, such as fibres. The reduced width helps to reducethe insertion loss of the waveguide. Typical widths are about W₂=8 μm tocouple to standard single mode fibre and about W₁=4 μm for the waveguidewidth. Any symmetry of the structure shown can be used.

[0250]FIG. 30 illustrates an alternative transition waveguide section130 which has curved sides, rather than straight as in the trapezoidaltransition section disclosed in FIG. 29. In FIG. 30, the curved sidesare shown as sections of circles of radius R₁ and R₂, subtending anglesφ₁ and φ₂ respectively, but it should be appreciated that variousfunctions can be implemented, such as exponential or parabolic, suchthat the input/output reflections and the transition losses areminimised.

[0251]FIG. 31 shows a curved waveguide section 108 which can be used toredirect the plasmon-polariton wave. The angle φ of the bend can be anyangle suitable for a particular implementation.

[0252] In addition to the width and thickness of the strip, criticaldimensions in the design of a bend include the bending radius R andoffsets O₁ and O₂ determining the positions of the input and outputstraight sections 100. Although the bend will work for many stripdimensions and bend designs, i.e., the structure 108 will convey theplasmon-polariton wave around the bend, there may be (i) radiation lossor leakage out of the bend, (ii) transition losses at the planes wherethe bent section meets the straight sections, and possibly also (iii)reflections back towards the direction from which the wave came. Minimalradiation loss or leakage is obtained by selecting a radius of curvaturefor the strip that is greater than a threshold value, for whichradiation is negligible.

[0253] Reduced transition and reflection losses may be obtained byoffsetting the input and output waveguides 100 radically outwardsrelative to the ends of the bend. The reason for this is that thestraight waveguide sections 100 have an optical field extremum thatpeaks along the longitudinal centre line of the strip, and decaystowards its edges. In the bend, the extremum of the optical fielddistribution shifts towards the exterior of the bend. This results inincreased radiation or leakage from the bend and increased transitionand reflection losses due to a mismatch in the field distributions.Offsetting the input and output waveguides 100 towards the outside ofthe bend aligns the extrema of their optical fields more closely withthat of the optical field in the bent section 108, which helps toreduce, even minimise, both transition and reflection losses. Thesmaller the radius R, the greater the radiation from the bend. Also theoffsets O₁ and O₂ are related to the radius R. Optimum values for theseparameters would have to be determined according to the specific bendingrequirements.

[0254] It should also be noted that it is not necessary to connect theinput and output waveguides 100 directly to the curved section. As shownin FIG. 31, it is possible to have a short spacing d₁ between the end ofthe input waveguide 100 and the adjacent end of the curved section 108.Generally speaking, that spacing d₁ should be minimised, even zero, andprobably no more than a few optical wavelengths. A similar offset O₂ andspacing d₂ could be provided between the bend 108 and the outputstraight waveguide 100.

[0255] Although FIG. 31 shows no gradual transition between the straightwaveguides 100 at the input and output and the ends of the curvedsection 108, it is envisaged that, in practice, a more gradual offsetcould be provided so as to reduce edge effects at the corners.

[0256] Estimates of radiation loss associated with 90° bends wereobtained as a function of the bend's radius of curvature R for variousstrip widths and thickness waveguide bends comprised of a gold (Au)strip, and for two cases of surrounding material: silicon dioxide (SiO₂)and lithium niobate (LiNbO3). The results clearly indicated the onset ofsignificant radiation loss as the radius of curvature for a particularstrip geometry was reduced. It was inferred from the results that goodbending radii are in the range from about 100 μm to about 10 cm for astrip width in the range from about 0.1 μm to about 12 μm and a stripthickness in the range from about 5 nm to about 100 nm.

[0257] Acceptable 90 degree bends, comprising strips surrounded bymaterials such as silicon dioxide, silicon oxynitride or siliconnitride, having a refractive index in the range from about 1.4 to about2.0, the range of dimensions for the metallic strip being a width in therange from about 0.5 μm to about 12 μm and a thickness in the range fromabout 5 nm to about 50 nm should have a radius of curvature in the rangefrom about 100 μm to about 10 cm. Such bends support effectively thepropagation of a plasmon-polariton wave having a wavelength in the rangefrom about 0.8 μm to about 2 μm. For operation in the wavelength rangefrom about 1.3 μm to about 1.7 μm, 90 degree bends have the followingdimensions for the metallic strip: a width in the range from about 0.7μm to about 8 μm, a thickness in the range from about 15 nm to about 25nm and a radius of curvature in the range from about 1 mm to about 10cm. For operation at a wavelength near 1.55 μm, a particularly goodchoice for a 90 degree bend is a metallic strip having a width of about6 μm, a thickness of about 20 nm and a radius of about 2 cm. Othermaterials having an index of refraction approximately in the same rangerequire strip dimensions and radii of curvature in approximately thesame ranges.

[0258] For 90 degree bends comprising strips surrounded by materialssuch as lithium niobate or PLZT, having a refractive index in the rangefrom about 2 to about 2.5, required to support effectively thepropagation of a plasmon-polariton wave having a wavelength in the rangefrom about 0.8 μm to about 2 μm, the ranges of dimensions for themetallic strip include width in the range from about 0.15 μm to about 6μm, thickness in the range from about 5 nm to about 80 nm, and radius ofcurvature in the range from about 100 μm to about 10 cm. For operationin the wavelength range from about 1.3 μm to about 1.7 μm, 90 degreebends preferably have the following dimensions for the metallic strip: awidth in the range from about 0.4 μm to about 2 μm, a thickness in therange from about 15 nm to about 40 nm and a radius of curvature in therange from about 1 mm to about 10 cm. A particularly good choice for a90 degree bend is a metallic strip having a width of about 1.5 μm, athickness of about 20 nm and a radius of about 4 cm for operation at awavelength near 1.55 μm. Other materials having an index of refractionapproximately in the same range require strip dimensions and radii ofcurvature in approximately the same ranges. Similar ranges apply tosemiconductors having a refractive index in the range from about 2.5 toabout 3.5, such as silicon, gallium arsenide and indium phosphide.

[0259] The radius of curvature selected can be smaller than that usedfor a 90 degree bend if the desired bending angle is smaller than 90degrees.

[0260]FIG. 32 shows a two-way power splitter 110 formed from atrapezoidal section 106 with a straight section 100 coupled to itsnarrower end 112 and two angled sections 104 coupled side-by-side to itswider end 114. The distances between the input waveguide 100 and thenarrower end 112 of the tapered section 106, and between the outputwaveguides 104 and the wider end 114 of the tapered section 106, d₁, d₂and d₃, respectively, should be minimised. The angle between the outputwaveguides 104 can be in the range of 0.5 to 10 degrees and their widthsare usually similar. The offsets S₁ and S₂ between the output waveguidesand the longitudinal centre line of the trapezoidal section 106preferably are set to zero, but could be non-zero, if desired, and varyin size. Ideally, however, the output sections 104 should together beequal in width to the wider end 114.

[0261] Offset S₁ need not be equal to offset S₂ but it is preferablethat both are set to zero. The widths of the output sections 104 can beadjusted to vary the ratio of the output powers. The dimensions of thecentre tapered section 106 are usually adjusted to minimise input andoutput reflections and radiation losses in the region between the outputsections 104.

[0262] It should also be noted that the centre tapered section 106 couldhave angles that vary according to application and need not besymmetrical.

[0263] It is envisaged that the tapered section 106 could be replaced bya rectangular transition section having a width broader than the widthof the input waveguide 100 so that the transition section favouredmultimode propagation causing constructive and destructive interferencepatterns throughout its length. The length could be selected so that, atthe output end of the rectangular transition section, the constructiveportions of the interference pattern would be coupled into the differentwaveguides establishing, in effect, a 1-to-N power split. Such asplitter then would be termed a multimode interferometer-based powerdivider.

[0264] It should be appreciated that the device shown in FIG. 32 couldalso be used as a combiner. In this usage, the light would be injectedinto the waveguide sections 104 and combined by the tapered centresection 106 to form the output wave which would emerge from the straightwaveguide section 100.

[0265] In either the Y splitter or the interferometer-based powerdivider, the number of arms or limbs 104 at the output could be far morethan the two that are shown in FIG. 32.

[0266] It is also feasible to have a plurality of input waveguides. Thiswould enable an N×N divider to be constructed. The dimensions of thetransition section 106 then would be controlled according to the type ofsplitting/combining required.

[0267] As shown in FIG. 33(a), an angled waveguide section 104 may beused to form an intersection between two straight waveguide sections100, with the dimensions adjusted for the particular application. Itshould be noted that, as shown in FIG. 33(a), the two straight sections100 are offset laterally away from each other by the distances O₁ andO₂, respectively, which would be selected to optimise the couplings byreducing radiation, transition and reflection losses. The angle of thetrapezoidal section 104 relative to the sections 100 will be a factor indetermining the best values for the offsets O₁ and O₂. The sections 100and 104 need not be connected directly together, but could be spaced bythe distances d₁ and d₂ and/or coupled by a suitable transition piecethat would make the junction more gradual (i.e., the change of directionwould be more gradual).

[0268] The embodiments of FIGS. 31 and 33(a) illustrate a generalprinciple of aligning optical fields, conveniently by offsets, whereverthere is a transition or change of direction of the optical wave and aninclination relative to the original path, which can cause radiation,transition and reflection losses if fields are misaligned. Such offsetscould be applied whether the direction-changing sections were straightor curved.

[0269] As shown in FIG. 33(b), an alternative embodiment to that shownin FIG. 33(a) is obtained by replacing the angled section 104 by a pairof concatenated bend sections 108 arranged to form an S bend (withoffsets) in order to provide the interconnection between the sections100. The bend sections could be replaced with bend sections having acontinuously variable radius of curvature implementing a bend profileother than circular.

[0270] As illustrated in FIG. 34(a), a power divider 116 can also beimplemented using a pair of concatenated bend sections 108 instead ofeach of the angled sections 104 in the splitter 110 shown in FIG. 32. Asshown in FIG. 34(a), in each pair, the bend section nearest to the widerend 114 of the tapered section 106 bends outwards from the longitudinalcentre line of the tapered section 106 while the other section bendsoppositely so that they form an “S” bend. Also, the bend sections ineach pair are offset by distance O₁ or O₂ one relative to the other forthe reasons discussed with respect to the bend 108 shown in FIG. 31.Other observations made regarding the power divider and the bend sectiondisclosed in FIGS. 31 and 32 respectively, also apply to this case.

[0271]FIG. 34(b) shows a modification to the power divider of FIG. 34(a)specifically the use of a bifurcated transition section 106′ having anarrower end portion 115A and two curved sections 115B and 115Cdiverging away from the narrower end portion 115A; in effect, thetransition section comprising two mirrored and overlapping bendsections. The light emerges from respective distal ends of the curvedsections 115B and 115C. Two additional bend sections 108 are shownconnected to distal ends of the two curved sections 115B and 115C,respectively, in effect forming with the transition section 106′ twomirrored:and overlapping S-bends.

[0272]FIG. 35 illustrates a Mach-Zender interferometer 118 created byinterconnecting two power splitters 110 as disclosed in FIG. 32. Ofcourse, either or both of them could be replaced by the power splitter116 shown in FIG. 34(a) or FIG. 34(b). Light injected into one of theports, i.e. the straight section 100 of one power splitter 110/116, issplit into equal amplitude and phase components that travel along theangled arms 104 of the splitter, are coupled by straight sections 100into the corresponding arms of the other splitter, and then arerecombined to form the output wave.

[0273] If the insertion phase along one or both arms of the device ismodified, then destructive interference between the re-combined wavescan be induced. This induced destructive interference is the basis of adevice that can be used to modulate the intensity of an input opticalwave. The lengths of the arms 100 are usually adjusted such that thephase difference in the re-combined waves is 180 degrees for aparticular relative change in insertion phase per unit length along thearms. The structure will thus be optically long if the mechanism used tomodify the per unit length insertion phase is weak (or optically shortif the mechanism is strong).

[0274]FIG. 36(a) illustrates a modulator 120 based on the Mach-Zender118 disclosed in FIG. 35. As illustrated also in FIG. 36(b), parallelplate electrodes 122 and 124 are disposed above and below, respectively,each of the strips 100 which interconnects two angled sections 104, andspaced from it, by the dielectric material, at a distance large enoughthat optical coupling to the electrodes is negligible. The electrodesare connected in common to one terminal of a voltage source 126, and theintervening strip 100 is connected using a minimally invasive contact tothe other terminal. Variation of the voltage V applied by source 126effects the modulating action. According to the plasma model for thestrip 100, a change in the carrier density of the latter (due tocharging +2Q or −2Q) causes a change in its permittivity, which in turncauses a change in the insertion phase of the arm. (The change inducedin the permittivity is described by the plasma model representing theguiding strip 100 at the operating wavelength of interest. Such model iswell known to those of ordinary skill in the art and so will not bedescribed further herein. For more information the reader is directed toreference [21], for example.) This change is sufficient to inducedestructive interference when the waves in both arms re-combine at theoutput combiner.

[0275]FIG. 36(c) illustrates an alternative connection arrangement inwhich the two plate electrodes 122 and 124 are connected to respectiveones of the terminals of the voltage source 126. In this case, thedielectric material used as the background of the waveguide iselectro-optic (LiNbO₃, a polymer, . . . ). In this instance, the appliedvoltage V effects a change in the permittivity of the backgrounddielectric, thus changing the insertion phase along the arm. This changeis sufficient to induce destructive interference when the waves in botharms re-combine at the output combiner.

[0276] It will be noted that, in FIG. 36(a), one voltage source suppliesthe voltage V₁ while the other supplies the voltage V₂. V₁ and V₂ may ormay not be equal.

[0277] For both cases described above, it is possible to apply voltagesin opposite polarity to both arms of the structure. This effects anincrease in the insertion phase of one arm and a decrease in theinsertion phase of the other arm of the Mach-Zender (or vice versa),thus reducing the magnitude of the voltage or the length of thestructure required to achieve a desirable degree of destructiveinterference at the output.

[0278] Also, it is possible to provide electrodes 122 and 124 and asource 126 for only one of the intervening strips 100 in order toprovide the required interference.

[0279] It should be appreciated that other electrode structures could beused to apply the necessary voltages. For example, the electrodes 122and 124 could be coplanar with the intervening strip 100, one on eachside of it. By carefully laying out the electrodes as a microwavewaveguide, a high frequency modulator (capable of modulation rates inexcess of 10 Gbit/s) can be achieved.

[0280]FIG. 37 illustrates an alternative implementation of a Mach-Zender128 which has the same set of waveguides as that shown in FIG. 35 butwhich makes use of magnetic fields B applied to either or both of themiddle straight section arms to induce a change in the permittivitytensor describing the strips. (The change induced in the tensor isdescribed by the plasma model representing the guiding strip at theoperating wavelength of interest. Such model is well known to those ofordinary skill in the art and so will not be described further herein.For more information the reader is directed to reference [21], forexample.) The change induced in the permittivity tensor will induce achange in the insertion phase of either or both arms thus inducing arelative phase difference between the light passing in the arms andgenerating destructive interference when the waves re-combine at theoutput combiner. Modulating the magnetic field thus modulates theintensity of the light transmitted through the device. The magneticfield B can be made to originate from current-carrying wires or coilsdisposed around the arms 100 in such a manner as to create the magneticfield in the desired orientation and intensity in the opticalwaveguides. The magnetic field may have one or all of the orientationsshown, B_(x), B_(y) or B_(z) or their opposites. The wires or coilscould be fabricated using plated via holes and printed lines or otherconductors in known manner. Alternatively, the field could be providedby an external source, such as a solenoid or toroid having poles on oneor both sides of the strip.

[0281]FIG. 38 illustrates a periodic waveguide structure 132 comprisinga series of unit cells 134, where each cell 134 comprises tworectangular waveguides 100 and 100′ having different lengths l₁ and l₂and widths w₁ and w₂, respectively. The dimensions of the waveguides ineach unit cell 134, the spacing d₁ therebetween, the number of unitcells, and the spacings d₂ between cells are adjusted such thatreflection occurs at a desired operating wavelength or over a desiredoperating bandwidth for an optical signal propagating along the gratingaxis, i.e. the longitudinal axis of the array of cells 134. The periodof the periodic structure, i.e. the length of each unit cell,l₁+l₂+d₁+d₂, can be made optically long, such that a long-periodperiodic structure is obtained. The dimensions of the elements 100, 100′in each unit cell 134 can also be made to change along the direction ofthe periodic structure in order to implement a prescribed transfer,function (like in a chirped periodic structure).

[0282] It should be noted that the waveguides 100, 100′ in each cellneed not be rectangular, but a variety of other shapes could be used.For example, FIG. 39 illustrates a portion, specifically two unit cells138 only, of an alternative periodic structure 136 in which each unitcell 138 comprises two of the trapezoidal waveguide sections 106, 106′like that described with reference to FIG. 30, with their wider edgesopposed.

[0283] As another alternative, the trapezoidal waveguides 106/106′ couldbe replaced by the transition sections 130, shown in FIG. 30, with orwithout spacings d₁ and d₂, to form a periodic structure havingsinusoidally-varying sides. It should be noted that these periodicstructures are merely examples and not intended to provide an exhaustivedetailing of all possibilities; various other periodic structures couldbe formed from unit cells comprised of all sorts of different shapes andsizes of elements.

[0284] It should be noted that voltages can be applied to some or all ofthe strips in order to establish charges on the strips of the unitcells, which would change their permittivity and thus vary the opticaltransfer function of the periodic structures. If the dielectric materialsurrounding the strip is electro-optic, then the applied voltages wouldalso change the permittivity of the dielectric, which also contributesto changing of the optical transfer function of the periodic structure.

[0285] A two-dimensional photonic bandgap structure can be created byplacing two or more of the periodic structures side-by-side to form atwo-dimensional array of unit cells (comprised of strips of variousshapes and sizes). A three-dimensional array can be created by stackinga plurality of such two-dimensional arrays in numerous planes separatedby dielectric material. The size and shape of the strips are determinedsuch that stop bands in the optical spectrum appear at desired spectrallocations.

[0286]FIG. 40(a) illustrates an edge coupler 139 created by placing twostrips 100″ parallel to each other and in close proximity over a certainlength. The separation S_(c) between the strips 100″ could be from about1 μm (or less) to about 20 μm and the coupling length L_(c) could be inthe range of a few microns to a few dozen millimeters depending on theseparation S_(c), width and thickness of the strips 100″, the materialsused, the operating wavelength, and the level of coupling desired. Sucha positioning of the strips 100″ is termed “edge coupling”. For example,if the strips are Au, the surrounding material is SiO₂ and the operatingfree space wavelength is near 1550 nm, good coupling lengths L_(c) arein the range from about 100 μm to about 5 mm and good separations S_(c)are in the range from about 2 μm to about 20 μm. Specific dimensionsdepend on the thickness and width of the strips and are selected toimplement a desired coupling factor.

[0287] The gaps between the input and output of the waveguide sectionsshown would ideally be set to zero and a lateral offset provided betweensections where a change of direction is involved. As shown in FIG.40(c), concatenated bend sections 108 forming an S bend, as discussed inthe embodiments associated with FIGS. 33 and 34 could be used instead ofthe sections 104, 100 and 100″ shown in FIG. 40(a). Other observationsmade regarding the bend section disclosed in FIG. 31 also hold in thiscase.

[0288] Although only two strips 100″ are shown in the coupled section,it should be understood that more than two strips can be coupledtogether to create an N×N coupler.

[0289] As illustrated in FIG. 40(b) a voltage can be applied to the twoedge-coupled sections 100″ via minimally invasive electrical contacts.FIG. 40(b) shows a voltage source 126 connected directly to the sections100″ but, if the sections 100, 104 and 100″ in each arm are connectedtogether electrically, the source 126 could be connected to one of theother sections in the same arm. Applying a voltage in such a mannercharges the arms of the coupler, which, according to the plasma modelfor the waveguide, changes its permittivity. If, in addition, thedielectric material placed between the two waveguides 100 iselectro-optic, then a change in the background permittivity will also beeffected as a result of the applied voltage. The first effect issufficient to change the coupling characteristics of the structure but,if an electro-optic dielectric is also used, as suggested, then botheffects will be present, allowing the coupling characteristics to bemodified by applying a lower voltage.

[0290] FIGS. 41(a) and 41(b) illustrate coupled waveguides similar tothose shown in FIG. 40(a) but placed on separate layers in a substratehaving several layers 140/1, 140/2 and 140/3. The strips could be placedone directly above the other with a thin region of dielectric ofthickness d placed between them. Such positioning of the strips istermed “broadside coupling”. The coupled guides can also be offset frombroadside a distance S_(c), as shown in FIGS. 41(a) and 41(b). Thestrips could be separated by about d=1 μm (or less) to about 20 μm, thecoupling length could be in the range of a few microns to a few dozensmillimeters and the separation S_(c) could be in the range of about −20to about +20 μm, depending on the width and thickness of the strips, thematerials used and the level of coupling desired. (The negativeindicates an overlap condition).

[0291] As before, curved sections could be used instead of the straightand angled sections shown in FIG. 41(a).

[0292] Gaps can be introduced longitudinally between the segments ofstrip if desired and a lateral offset between the straight and angled(or curved) sections could be introduced.

[0293] Though only two strips are shown in the coupled section, itshould be understood that a plurality of strips can be coupled togetheron a layer and/or over many layers to create an N×N coupler.

[0294] As shown in FIG. 41(b), a voltage source 126 could be connecteddirectly or indirectly to the middle (coupled) sections 100″ in asimilar manner to that shown in FIG. 40(b).

[0295] It should be appreciated that in any of the couplers describedwith reference to FIGS. 40(a), 40(c) and 41(a), the adjusting meansshown in FIG. 40(b) could be replaced by either of the electrodeconfigurations and biasing arrangements described with reference toFIGS. 36(b) and 36(c). It should also be noted that the electrodeconfiguration and biasing arrangement could be applied to either or bothof strips 100″ of such couplers.

[0296] As illustrated in FIG. 42, an intersection 142 can be created byconnecting together respective ends of four of the angled waveguidesections 104, their distal ends providing input and output ports for thedevice. When light is applied to one of the ports, a prescribed ratio ofoptical power emerges from the output ports at the opposite side of theintersection. The angles φ₁ . . . φ₄ can be set such that optical powerinput into one of the ports emerges from the port directly opposite,with negligible power transmitted out of the other ports. Any symmetryof the structure shown is appropriate.

[0297] Various other modifications and substitutions are possiblewithout departing from the scope of the present invention. For example,although the waveguide structure shown in FIGS. 1(a) and 1(b), andimplicitly those shown in other Figures, have a single homogeneousdielectric surrounding a thin metal film, it would be possible tosandwich the metal film between two slabs of different dielectricmaterial; or at the junction between four slabs of different dielectricmaterial. Moreover, the multilayer dielectric material(s) illustrated inFIG. 41(a) could be used for other devices too. Also, the thin metalfilm could be replaced by some other conductive material or a highly n-or p-doped semiconductor. It is also envisaged that the conductive film,whether metal or other material, could be multi-layered.

[0298] Specific Embodiments of Modulation and Switching Devices

[0299] Modulation and switching devices will now be described which makeuse of the fact that an asymmetry induced in optical waveguidingstructures having as a guiding element a thin narrow metal film mayinhibit propagation of the main long-ranging purely boundplasmon-polariton mode supported.

[0300] The asymmetry in the structure can be in the dielectricssurrounding the metal film. In this case the permittivity, permeabilityor the dimensions of the dielectrics surrounding the strip can bedifferent. A noteworthy case is where the dielectrics above and belowthe metal strip have different permittivities, in a manner similar tothat shown in FIG. 17(a).

[0301] A dielectric material exhibiting an electro-optic, magneto-optic,thermo-optic, or piezo-optic effect can be used as the surroundingdielectric medium. The modulation and switching devices make use of anexternal stimulus to induce or enhance the asymmetry in the dielectricsof the structure.

[0302] FIGS. 43(a) and 43(b) depict an electro-optic modulatorcomprising two metal strips 110 and 112 surrounded by a dielectric 114exhibiting an electro-optic effect. Such a dielectric has a permittivitythat varies with the strength of an applied electric field. The effectcan be first order in the electric field, in which case it is termed thePockels effect, or second order in the electric field (Kerr effect), orhigher order. FIG. 43(a) shows the structure in cross-sectional view andFIG. 43(b) shows the structure in top view. The lower metal strip 110and the surrounding dielectric 114 form the optical waveguide. The lowermetal strip 110 is dimensioned such that a purely bound long-rangingplasmon-polariton wave is guided by the structure at the opticalwavelength of interest. Since the “guiding” lower metal strip 110comprises a metal, it is also used as an electrode and is connected to avoltage source 116 via a minimally invasive electrical contact 118 asshown. The second metal strip 112 is positioned above the lower metalstrip 110 at a distance large enough that optical coupling between thestrips is negligible. It is noted that the second strip also be placedbelow the waveguiding strip instead of above. The second strip acts as asecond electrode.

[0303] The intensity of the optical signal output from the waveguide canbe varied or modulated by varying the intensity of the voltage V appliedby the source 116. When no voltage is applied, the waveguiding structureis symmetrical and supports a plasmon-polariton wave. When a voltage isapplied, an asymmetry in the waveguiding structure is induced via theelectro-optic effect present in the dielectric 114, and the propagationof the plasmon-polariton wave is inhibited. The degree of asymmetryinduced may be large enough to completely cut-off the main purely boundlong-ranging mode, thus enabling a very high modulation depth to beachieved. By carefully laying out the electrodes as a microwavewaveguide, a high frequency modulator (capable of modulation rates inexcess of 10 G bit/s) can be achieved.

[0304] FIGS. 44(a) and 44(b) show an alternative design for anelectro-optic modulator which is similar to that shown in FIG. 43(a) butcomprises electrodes 112A and 112B placed above and below, respectively,the metal film 110 of the optical waveguide at such a distance thatoptical coupling between the strips is negligible. FIG. 44(a) shows thestructure in cross-sectional view and FIG. 44(b) shows the structure intop view. A first voltage source 116A connected to the metal film 110and the upper electrode 112A applies a first voltage V₁ between them. Asecond voltage source 116B connected to metal film 110 and lowerelectrode 112B applies a voltage V₂ between them. The voltages V₁ andV₂, which are variable, produce electric fields E₁ and E₂ in portions114A and 114B of the dielectric material. The dielectric material usedexhibits an electro-optic effect. The waveguide structure shown in FIG.44(c) is similar in construction to that shown in FIG. 44(a) but onlyone voltage source 116C is used. The positive terminal (+) of thevoltage source 116C is shown connected to metal film 110 while itsnegative terminal (−) is shown connected to both the upper electrode112A and the lower electrode 112B. With this configuration, the electricfields E₁ and E₂ produced in the dielectric portions 114A and 114B,respectively, are in opposite directions. Thus, whereas, in thewaveguide structure of FIG. 44(a), selecting appropriate values for thevoltages V₁ and V₂ induces the desired asymmetry in the waveguidestructure of FIG. 44(c), the asymmetry is induced by the relativedirection of the electric field above and below the waveguiding strip110, since the voltage V applied to the electrodes 112A and 112Bproduces electric fields acting in opposite directions in the portions114A and 114B of the dielectric material.

[0305] The structures shown in FIGS. 44(a),(b) and (c) can operate tovery high frequencies since a microwave transmission line (a stripline)is in effect created by the three metals.

[0306]FIG. 45 shows in cross-sectional view yet another design for anelectro-optic modulator. In this case, the metal film 110 is embedded inthe middle of dielectric material 114 with first portion 114D above itand second portion 114E below it. Electrodes 112D and 112E are placedopposite lateral along opposite lateral edges, respectively, of theupper portion 114D of the dielectric 114 as shown and connected tovoltage source 116E which applies a voltage between them to induce thedesired asymmetry in the structure. Alternatively, the electrodes 112D,112E could be placed laterally along the bottom portion 114E of thedielectric 114, the distinct portions of the dielectric material stillproviding the asymmetry being above and below the strip.

[0307]FIG. 46 shows an example of a magneto-optic modulator wherein thewaveguiding strip 110 and overlying electrode 112F are used to carry acurrent I in the opposite directions shown. The dielectric materialsurrounding the metal waveguide strip 110 exhibits a magneto-opticeffect or is a ferrite. The magnetic fields generated by the current Iadd in the dielectric portion between the electrodes 110 and 112F andessentially cancel in the portions above the top electrode 112F andbelow the waveguide 110. The applied magnetic field thus induces thedesired asymmetry in the waveguiding structure. The electrode 112F isplaced far enough from the guiding strip 110 that optical couplingbetween the strips is negligible.

[0308]FIG. 47 depicts a thermo-optic modulator wherein the waveguidingstrip 110 and the overlying electrode 112G are maintained attemperatures T₂ and T₁ respectively. The dielectric material 114surrounding the metal waveguide exhibits a thermo-optic effect. Thetemperature difference creates a thermal gradient in the dielectricportion 114G between the electrode 112G and the strip 110. The variationin the applied temperature thus induce the desired asymmetry in thewaveguiding structure. The electrode 112G is placed far enough from theguiding strip 110 that optical coupling between the strips isnegligible.

[0309] It should be appreciated that the modulator devices describedabove with reference to FIGS. 43(a) to 47 may also serve as variableoptical attenuators with the attenuation being controlled via theexternal stimulus, i.e. voltage, current, temperature, which varies theelectromagnetic property.

[0310]FIGS. 48, 49 and 50 depict optical switches that operate on theprinciple of “split and attenuate”. In each case, the input opticalsignal is first split into N outputs using a power divider; a one-to-twopower split being shown in FIGS. 48, 49 and 50. The undesired outputsare then “switched off” or highly attenuated by inducing a largeasymmetry in the corresponding output waveguides. The asymmetry must belarge enough to completely cut-off the main purely bound long-rangingmode supported by the waveguides. The asymmetry is induced by means ofoverlying electrodes as in the waveguide structures of FIGS. 43, 46 or47, respectively.

[0311] In the switches shown in FIGS. 48, 49 and 50, the basic waveguideconfiguration is the same and comprises an input waveguide section 120coupled to two parallel branch sections 122A and 122B by a wedge-shapedsplitter 124. All four sections 120, 122A, 122B and 124 are co-planarand embedded in dielectric material 126. The thickness of the metal filmis d₃. Two rectangular electrodes 128A and 128B, each of thickness d₁,are disposed above branch sections 122A and 122B, respectively, andspaced from them by a thickness d₂ of the dielectric material 126 at adistance large enough that optical coupling between the strips isnegligible. Each of the electrodes 128A and 128B is wider and shorterthan the underlying metal film 122A or 122B, respectively. In the switchshown in FIG. 48, the asymmetry is induced electro-optically by means ofa first voltage source 130A connected between metal film 122A andelectrode 128A for applying voltage V₁ therebetween, and a secondvoltage source 130B connected between metal film 122B and electrode128B, for applying a second voltage V₂ therebetween. In the switch shownin FIG. 49, the asymmetry is induced magneto-optically by a firstcurrent source 132A connected between metal film 122A and electrode128A, which are connected together by connector 134A to complete thecircuit, and a second current source 132B connected between metal film122B and electrode 128B, which are connected together by connector 134Bto complete that circuit.

[0312] In the switch shown in FIG. 50, the asymmetry is inducedthermo-optically by maintaining the metal strips 122A and 122B attemperature T₀ and the overlying electrodes 128A and 128B attemperatures T₁ and T₂, respectively.

[0313] It will be appreciated that, in the structures shown in FIGS. 48,49 and 50, the dielectric surrounding the metal strip will beelectro-optic, magneto-optic, or thermo-optic, or a magnetic materialsuch as a ferrite, as appropriate.

[0314] In general, any of the sources, whether of voltage, current ortemperature, may be variable.

[0315] Although the switches shown in FIGS. 48, 49 and 50 are 1×2switches, the invention embraces 1×N switches which can be created byadding more branch sections and associated electrodes, etc.

[0316] It will be appreciated that, where the surrounding material isacousto-optic, the external stimulus used to induce or enhance theasymmetry could be determined by analogy. For example, a structuresimilar to that shown in FIG. 45 could be used with the electro-opticmaterial replaced by acousto-optic material and the electrodes 112D and112E used to apply compression or tension to the upper portion 114D.

[0317] To facilitate description, the various devices embodying theinvention have been shown and described as comprising several separatesections of the novel waveguide structure. While it would be feasible toconstruct devices in this way, in practice, the devices are likely tocomprise continuous strips of metal or other high charge carrier densitymaterial, i.e. integral strip sections, fabricated on the samesubstrate.

[0318] The foregoing examples are not meant to be an exhaustive listingof all that is possible but rather to demonstrate the breadth ofapplication of the invention. The inventive concept can be applied tovarious other elements suitable for integrated optics devices. It isalso envisaged that waveguide structures embodying the invention couldbe applied to multiplexers and demultiplexers.

[0319] Although embodiments of the invention have been described andillustrated in detail, it is to be clearly understood that the same isby way of illustration and example only and not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the appended claims.

[0320] References

[0321] 1. American Institute of Physics Handbook, third edition.McGraw-Hill Book Company, 1972.

[0322] 2. Handbook of Optics. McGraw-Hill Book Company, 1978.

[0323] 3. NASH, D. J., SAMBLES, J. R. “Surface Plasmon-Polariton Studyof the Optical Dielectric Function of Silver”, Journal of Modern Optics,Vol. 43, No. 1 (1996), pp. 81-91.

[0324] 4. BOARDMAN, A. D., Editor. Electromagnetic Surface Modes. WileyInterscience, 1982.

[0325] 5. ECONOMOU, E. N. “Surface Plasmons in Thin Films”, PhysicalReview, Vol. 182, No. 2 (June 1969), pp. 539-554.

[0326] 6. BURKE, J. J., STEGEMAN, G. I., TAMIR, T.“Surface-Polariton-Like Waves Guided by Thin, Lossy Metal Films”,Physical Review B, Vol. 33, No. 8 (April 1986), pp. 5186-5201.

[0327] 7. WENDLER, L., HAUPT, R. “Lorig-Range Surface Plasmon-Polaritonsin Asymmetric Layer Structures”, Journal of Applied Physics, Vol. 59,No. 9 (May 1986), pp. 3289-3291.

[0328] 8. BURTON, F. A., CASSIDY, S. A. “A Complete Description of theDispersion Relation for Thin Metal Film Plasmon-Polaritons”, Journal ofLightwave Technology, Vol. 8, No. 12 (December 1990), pp. 1843-1849.

[0329] 9. PRADE, B., VINET, J. Y., MYSYROWICZ, A. “Guided Optical Wavesin Planar Heterostructures With Negative Dielectric Constant”, PhysicalReview B, Vol. 44, No. 24 (December 1991), pp. 13556-13572.

[0330] 10. TOURNOIS, P., LAUDE, V. “Negative Group Velocities inMetal-Film Optical Waveguides”, Optics Communications, April 1997, pp.41-45.

[0331] 11. JOHNSTONE, W., STEWART, G., HART, T., CULSHAW B. “SurfacePlasmon Polaritons in Thin Metal Films and Their Role in Fiber OpticPolarizing Devices”, Journal of Lightwave Technology, Vol. 8, No. 4(April 1990), pp. 538-544.

[0332] 12. RAJARAJAN, M., THEMISTOS, C., RAHMAN, B. M. A., GRATTAN, K.T. V. “Characterization of Metal-Clad TE/TM Mode Splitters Using theFinite Element Method”, Journal of Lightwave Technology, Vol. 15, No. 12(December 1997), pp. 2264-2269.

[0333] 13. BERINI, P. “Plasmon-Polariton Modes Guided by a Metal Film ofFinite Width”, Optics Letters, Vol. 24, No. 15 (August 1999), pp.1011-1013.

[0334] 14. PREGLA, R., PASCHER, W. “The Method of Lines”, NumericalTechniques for Microwave and Millimeter-Wave Passive Structures; WileyInterscience, 1989. T. ITOH, Editor.

[0335] 15. BERINI, P., WU, K. “Modeling Lossy Anisotropic DielectricWaveguides With the Method of Lines”, IEEE Transactions on MicrowaveTheory and Techniques, Vol. MTT-44, No. 5 (May 1996), pp. 749-759.

[0336] 16. BERINI, P., STÖHR, A., WU, K., JÄGER, D. “Normal ModeAnalysis and Characterization of an InGaAs/GaAs MQW Field-InducedOptical Waveguide Including Electrode Effects”, Journal of LightwaveTechnology, Vol. 14, No. 10 (October 1996), pp. 2422-2435.

[0337] 17. CULVER, R. “The Use of Extrapolation Techniques WithElectrical Network Analogue Solutions”, British Journal of AppliedPhysics, Vol. 3 (December 1952), pp. 376-378.

[0338] 18. BOONTON, R. C. Computational Methods for Electromagnetics andMicrowaves. Wiley Interscience, 1992.

[0339] 19. STEGEMAN, G. I., WALLIS, R. F., MARADUDIN, A. A. “Excitationof Surface Polaritons by End-Fire Coupling”, Optics Letters, Vol. 8, No.7 (July 1983), pp. 386-388.

[0340] 20. BERINI, P. “Plasmon-Polariton Waves Guided by Thin LossyMetal Films of Finite Width: Bound Modes of Symmetric Structures”,Physical Review B, Vol. 61, No. 15, (2000), pp. 10484-10503.

[0341] 21. KRAUS, et al., Electromagnetics, second edition. McGraw Hill.

[0342] 22. CHARBONNEAU, R., BERINI, P., BEROLO, E., LISICKA-SKRZEK, E.,“Experimental Observation of Plasmon-Polariton Waves Supported by a ThinMetal Film of Finite Width”, Optics Letters, Vol. 25, No. 11, pp.844-846, June 2000.

[0343] 23. EVANS, A. F., HALL, D. G., “Measurement of the electricallyinduced refractive index change in silicon for wavelength λ=1.3 μm usinga Schottky diode” Applied Physics Letters, Vol. 56, No. 3, pp. 212-214,January 1990.

[0344] 24. JUNG, C., YEE, S., KUHN, K., “Integrated Optics WaveguideModulator Based on Surface Plasmons”, Journal of Lightwave Technology,Vol. 12, No. 10, pp. 1802-1806, October 1994.

[0345] 25. SOLGAARD, O., HO, F., THACKARA, J. I., BLOOM, D. M., “Highfrequency attenuated total internal reflection light modulator”, AppliedPhysics Letters, Vol. 61, No. 21, pp. 2500-2502, November 1992.

[0346] 26. SOLGAARD, O., et al., “Electro-optic Attenuated TotalInternal Reflection Modulator and Method”, U.S. Pat. No. 5,155,617,1992.

[0347] 27. SCHILDKRAUT, J. S., “Long-range surface plasmon electro-opticmodulator”, Applied Optics, Vol. 27, No. 21, pp. 4587-4590, November1988.

[0348] 28. SCHILDKRAUT, J. S., et al., “Optical Article for ReflectionModulation”, U.S. Pat. No. 5,157,541, 1992.

[0349] 29. SCHILDKRAUT, J. S., et al., “Optical Article for MulticolorImaging”, U.S. Pat. No. 5,075,796, 1991.

[0350] 30. SCHILDKRAUT, J. S., et al., “Optical Article for ReflectionModulation”, U.S. Pat. No. 4,971,426, 1990.

[0351] 31. RIDER, C. B., et al., “Nonlinear Optical Article forModulating Polarized Light”, U.S. Pat. No. 4,948,225, 1990.

[0352] 32. COLLINS, R. T., et al., “Optical Modulator”, U.S. Pat. No.4,915,482, 1990.

[0353] 33. McNEILL, W. H., et al., “High Frequency Light ModulationDevice”, U.S. Pat. No. 4,451,123, 1984.

[0354] 34. McNEILL, W. H., et al., “High Frequency Light Modulator”,U.S. Pat. No. 4,432,614, 1984.

[0355] 35. SINCERBOX, G. T., et al., “Projection Display Device”, U.S.Pat. No. 4,249,796, 1981.

[0356] 36. BROWN, T. G., “Optoelectronic Device for Coupling Between anExternal Optical Wave and a Local Optical Wave for Optical Modulatorsand Detectors”, U.S. Pat. No. 5,625,729, 1997.

[0357] 37. JANSSON, T. P., et al., “High Modulation Rate Optical PlasmonWaveguide Modulator”, U.S. Pat. No. 5,067,788, 1991.

[0358] 38. DRIESSEN, A., KLEIN KOERKAMP, H. M. M., POPMA, T H. J. A.,“Novel Integrated Optic Intensity Modulator Based on Mode Coupling”,Fibre and Integrated Optics, Vol. 13, pp. 445-461, 1994.

[0359] 39. HOEKSTRA, H. J. W. M., LAMBECK, P. V., KRIJNEN, G. J. M.,CTYROKY, J., De MINICIS, M., SIBILIA, C., CONRADI, O., HELFERT, S.,PREGLA, R., “A COST 240 Benchmark Test for Beam Propagation MethodsApplied to an Electrooptical Modulator Based on Surface Plasmons”,Journal of Lightwave Technology, Vol. 16, No. 10, pp. 1921-1926, October1998.

[0360] 40. ANEMOGIANNIS, E., “Optical Plasmon Wave Structures” U.S. Pat.No. 6,034,809, 2000.

[0361] 41. BERINI, P., “The Proximity Effect of Conductors in OpticalWaveguide Devices: Coupling to Plasmon-Polariton Modes”, SPIE SD-25Millimeter-Wave Materials Devices and Components, in print, July 2000.

[0362] 42. CHEN, Y. -J., et al., “Optical Device With Surface Plasmons”U.S. Pat. No. 4,583,818, 1986.

[0363] 43. BERINI, P., “Optical Waveguide Structures”, CopendingCanadian and U.S. patent applications.

[0364] 44. BERINI, P., “Plasmon-Polariton Modes Guided by a Metal Filmof Finite Width Bounded by Different Dielectrics”, Optics Express, Vol.7, No. 10, pp. 329-335.

[0365] 45. BERINI, P., “Plasmon-Polariton waves guided by thin lossymetal films of finite width: Bound Modes of Asymmetric Structures”,Physical Review B, Vol. 63, No. 12, March 2001.

What is claimed is:
 1. An optical device comprising a waveguide structure formed by a thin strip of material having a relatively high free charge carrier density surrounded by material having a relatively low free charge carrier density, the strip having finite width and thickness with dimensions such that optical radiation having a free space wavelength in the range from about 0.8 μm to about 2 μm couples to the strip and propagates along the length of the strip as a plasmon-polariton wave.
 2. A device according to claim 1, wherein the strip is straight, curved, bent, or tapered.
 3. A device according to claim 1, further comprising at least a second said waveguide structure similar to the first-mentioned waveguide structure, the second waveguide structure comprising a second said strip having one end coupled to the first-mentioned strip, wherein the first-mentioned strip is curved and the second strip is offset outwardly relative to an axis of curvature of the first-mentioned strip.
 4. A device according to claim 3, wherein the second strip is curved oppositely to the first strip such that the first and second strips form an S-bend.
 5. A device according to claim 4, further comprising third and fourth said waveguide structures comprising third and fourth strips, respectively, the third strip having one end coupled to an end of the first strip opposite that connected to the second strip and the fourth strip having one end coupled to an end of the second strip opposite that connected to the first strip, the third and fourth strips each being offset outwardly relative to an axis of curvature of the strip to which it is coupled.
 6. A device according to claim 1, further comprising at least two additional said waveguide structures comprising second and third strips, respectively, the second strip for inputting said radiation to one end of said first-mentioned strip and the third strip for receiving said radiation from the opposite end of said first-mentioned strip, wherein the first-mentioned strip is curved and said second and third strips are each offset outwardly relative to an axis of curvature of the first-mentioned strip.
 7. A device according to claim 1, further comprising at least second and third waveguide structures both similar in construction to the first-mentioned waveguide structure and having second and third strips, respectively, the second and third strips having respective ends coupled in common to one end of the strip of the first-mentioned waveguide structure to form respective arms of a combiner/splitter, the arrangement being such that said optical radiation leaving said first-mentioned strip via said one end will be split between said second and third strips and conversely said optical radiation coupled to said one end by said second and third strips will be combined to leave said first-mentioned strip by an opposite end.
 8. A device according to claim 7, further comprising a transition waveguide structure coupling the second and third strips to the first-mentioned strip, the transition waveguide structure being similar in construction to the first-mentioned waveguide structure and comprising a strip having a narrower end coupled to the first-mentioned strip and a wider end, the second and third strips being coupled together to the wider end.
 9. A device according to claim 8, wherein the second and third strips each comprise an S-bend, and the S-bends diverge away from the wider end of the transition strip.
 10. A device according to claim 1, further comprising a bifurcated transition section (106′) having a narrower end portion (115A) coupled to the first-mentioned waveguide structure and two curved sections (115B, 115C) diverging away from the narrower end portion 115A to form respective arms of a combiner/splitter, such that light entering the narrower end portion will emerge from respective distal ends of the curved sections 115B and 115C.
 11. A device according to claim 10, further comprising two additional bend sections (108) connected to distal ends of the curved sections (115B, 115C), respectively, to form with the transition section two mirrored and overlapping S-bends.
 12. A device according to any one of claims 7 to 11, further comprising a second combiner/splitter similar in construction to the first combiner/splitter and connected to the first combiner/splitter to form a Mach-Zender interferometer, each arm of the first combiner/splitter being connected to a respective one of the arms of the second combiner/splitter to form a corresponding interferometer arm, the arrangement being such that optical radiation input via said first strip of the first combiner/splitter produces two plasmon-polariton wave portions which propagate along, respectively, arms of the Mach-Zender interferometer and are recombined by the second combiner/splitter.
 13. A device according to claim 12, wherein each arm of the first combiner/splitter is connected to a respective arm of the second combiner/splitter by a respective intermediate section of waveguide structure similar in construction to the first waveguide structure, and the device further comprises means for adjusting the characteristics of one of said intermediate sections relative to those of the other intermediate section and thereby propagation characteristics of the corresponding one of said two plasmon-polariton wave portions so as to obtain destructive interference upon recombination and thereby modulate the intensity of said optical radiation.
 14. A device according to claim 13, wherein the material surrounding the strip of the waveguide structure whose characteristics are adjusted is electro-optic, and the adjusting means establishes an electric field in said electro-optic material and varies said electric field so as to vary the refractive index of the electro-optic material.
 15. A device according to claim 14, wherein the adjusting means comprises a pair of electrodes spaced apart with the strip between them and a voltage source connected to the electrodes for applying a voltage between the electrodes so as to establish said electric field in said electro-optical material.
 16. A device according to claim 14, wherein the adjusting means comprises at least one electrode adjacent the strip of the waveguide structure whose characteristics are adjusted and a voltage source for applying said voltage between the electrode and said strip so as to establish said electric field in said material therebetween.
 17. A device according to claim 14, wherein the adjusting means comprises a pair of electrodes spaced apart with said strip therebetween and a voltage source connected between said strip and both electrodes, in common, for applying a voltage between the electrodes and the strip to establish said electric field in said electro-optic material.
 18. A device according to claim 13, wherein the adjusting means is arranged to induce a magnetic field in said at least one of the parallel branches.
 19. A device according to claim 13, wherein the adjusting means modulates the intensity of said optical radiation substantially to extinction.
 20. A device according to claim 1, further comprising at least two additional waveguide structures, the at least three waveguide structures arranged to form an intersection, respective strips of the at least three waveguide structures each having one end connected to juxtaposed ends of the other strips to form said intersection, distal ends of the at least three strips constituting ports such that optical radiation input via the distal end of one of the strips will be conveyed across the intersection to emerge from at least one of the other strips.
 21. A device according to any one of claims 3 to 20, wherein said strips are integral with each other.
 22. A device according to claim 1, further comprising at least a second waveguide structure similar in construction to the first-mentioned waveguide structure, the first and second waveguide structures being arranged to form a coupler, first and second strips of the first and second waveguide structures, respectively, extending parallel to each other and in close proximity such that propagation of said optical radiation is supported by both strips, the device further comprising input means for inputting said optical radiation to at least said first strip and output means for receiving at least a portion of said optical radiation from at least said second strip.
 23. A device according to claim 22, wherein the first and second strips are not coplanar.
 24. A device according to claim 22 or 23, further comprising means for adjusting the characteristics of at least one of the first and second waveguide structures and thereby propagation characteristics of said plasmon-polariton wave propagating along the coupled strips so as to control the degree of coupling between the strips.
 25. A device according to claim 22 or 23, wherein the material between the coupled strips is electro-optic and further comprising adjusting means for establishing an electric field in said electro-optic material and varying said electric field so as to vary the refractive index of the electro-optic material between the strips.
 26. A device according to claim 25, wherein the adjusting means comprises a voltage source connected to the coupled strips for applying a voltage between the strips so as to establish said electric field in the said electro-optic material.
 27. A device according to claim 25, wherein the adjusting means comprises at least one electrode adjacent at least one of the coupled strips and a voltage source for applying a voltage between the electrode and at least one of the coupled strips so as to establish said electric field in said material therebetween.
 28. A device according to claim 22 or 23, wherein the material surrounding at least one of the coupled strips is electro-optic and the adjusting means comprises a pair of electrodes spaced apart with said at least one of the coupled strips between them and a voltage source connected to the electrodes for applying a voltage between the electrodes so as to establish an electric field in said electro-optic material, variation of said voltage causing a corresponding variation in the refractive index of said electro-optic material.
 29. A device according to claim 22 or 23, wherein the material surrounding at least one of the strips is electro-optic, and the adjusting means comprises a pair of electrodes spaced apart with said at least one of the coupled strips therebetween and a voltage source connected between said at least one of the strip and both electrodes in common, for applying a voltage between the electrodes and said at least one of the strips to establish an electric field in said electro-optic material, variation of said voltage causing a corresponding variation in the reflective index of said electro-optic material.
 30. A device according to any one of claims 22 to 29, wherein the input means and the output means each comprise a pair of waveguide structures similar in construction to the first waveguide structure, and each pair comprises a pair of strips connected at one end to the respective ends of the coupled strips and diverging away therefrom so that distal ends of each pair of strips are spaced apart by a distance significantly greater than the spacing between the coupled strips.
 31. A device according to claim 30, wherein each of said diverging strips comprises an S-bend.
 32. A device according to claim 1, wherein the surrounding material is inhomogeneous.
 33. A device according to claim 32, wherein the surrounding material comprises a continuously variable material composition or a combination of slabs and/or strips and/or laminae.
 34. A device according to claim 1, wherein said relatively low free charge carrier density is substantially negligible.
 35. A device according to claim 32, wherein the surrounding material comprises a single material or a combination of materials selected from the group consisting of glasses, electro-optic crystals, electro-optic polymers, and undoped or very lightly doped semiconductors, and preferably selected from the group consisting of silicon dioxide, silicon nitride, silicon oxynitride, quartz, lithium niobate, lead lanthanum zirconium titanate (PLZT), gallium arsenide, indium phosphide and silicon.
 36. A device according to claim 1 or 32, wherein the strip is inhomogeneous.
 37. A device according to claim 36, wherein the strip material comprises a single material or a combination of materials selected from the group consisting of metals, semi-metals, and highly n- or p-doped semiconductors, preferred materials being selected from the group consisting of gold, silver, copper, aluminium, platinum, palladium, titanium, nickel, molybdenum and chromium, metal silicides and highly n- or p-doped gallium arsenide (GaAs), indium phosphide (InP) or silicon (Si), and preferably from the subgroup consisting of gold (Au), silver (Ag), copper (Cu), and aluminium (Al), metal suicides such as cobalt disilicide (CoSi₂), and materials which behave like metals, such as Indium Tin Oxide (ITO).
 38. An optical device according to claim 37, wherein the strip comprises layers of different ones of said materials.
 39. A device according to claim 38, wherein the strip comprises a layer of gold and a layer of titanium and is surrounded by silicon dioxide.
 40. A device according to claim 37, wherein the strip comprises gold and is surrounded by silicon dioxide.
 41. A device according to claim 37, wherein the strip comprises gold and is surrounded by lithium niobate.
 42. A device according to claim 37 wherein the strip comprises gold and is surrounded by PLZT.
 43. A device according to claim 37, wherein the strip comprises gold and is surrounded by polymer.
 44. A device according to claim 37, wherein the strip comprises aluminium and is surrounded by silicon.
 45. A device according to claim 37, wherein the strip comprises cobalt disilicide and is surrounded by silicon.
 46. A device according to claim 1, wherein the width is in the range from about 0.1 microns to about 12 microns and the thickness is in the range from about 5 nm to about 100 nm.
 47. A device according to claim 46, wherein the surrounding material comprises silicon dioxide, silicon oxynitride, silicon nitride or other material having a refractive index in the range from about 1.4 to about 2.0, the width of the strip is in the range from about 0.5 μm to about 12 μm and the thickness of the strip is in the range from about 5 nm to about 50 nm.
 48. A device according to claim 47, wherein the width of the strip is in the range from about 0.7 μm to about 8 μm and the thickness of the strip is in the range from about 15 nm to about 25 nm, the device supporting propagation of a plasmon-polariton wave having a wavelength in the range from about 1.3 μm to about 1.7 μm.
 49. A device according to claim 48, wherein the width is about 4 μm and the thickness is about 20 nm, the device supporting propagation of a plasmon-polariton wave having a wavelength near 1.55 μm.
 50. A device according to claim 46, wherein the surrounding material comprises lithium niobate, PLZT or other material having refractive index in the range from about 2 to about 2.5, the width of the strip being in the range from about 0.15 μm to about 6 μm and the thickness of the strip being in the range from about 5 nm to about 80 nm.
 51. A device according to claim 50, wherein the width of the strip is in the range from about 0.4 μm to about 2 μm and the thickness of the strip is in the range from about 15 nm to about 40 nm, the device supporting propagation of a plasmon-polariton wave having a wavelength in the range from about 1.3 μm to about 1.7 μm.
 52. A device according to claim 51, wherein the width of the strip is about 1 μm and the thickness of the strip is about 20 nm, the device supporting propagation of a plasmon-polariton wave having a wavelength near 1.55 μm.
 53. A device according to claim 46, wherein the surrounding material comprises silicon, gallium arsenide, indium phosphide or other material having a refractive index in the range from about 2.5 to about 3.5, the width of the strip being in the range from about 0.1 μm to about 2 μm and the thickness of the strip being in the range from about 5 nm to about 30 nm.
 54. A device according to claim 53, wherein the width of the strip is in the range from about 0.13 μm to about 0.5 μm and the thickness of the strip is in the range from about 10 nm to about 20 nm, the device supporting propagation of a plasmon-polariton wave having a wavelength in the range from about 1.3 μm to about 1.7 μm.
 55. A device according to claim 54, wherein the width of the strip is about 0.25 μm and the thickness of the strip is about 15 nm, the device supporting propagation of a plasmon-polariton wave having a wavelength near 1.55 μm.
 56. A device according to claim 2, wherein the strip is curved, its width is in the range from about 0.1 microns to about 12 microns, its thickness is in the range from about 5 nm to about 100 nm, and its radius of curvature is in the range from about 100 microns to about 10 cm.
 57. A device according to claim 56, wherein the surrounding material comprises silicon dioxide, silicon oxynitride, silicon nitride, or other material having a refractive index in the range from about 1.4 to about 2.0, the width of the strip is in the range from about 0.5 μm to about 12 μm and the thickness of the strip is in the range from about 5 nm to about 50 nm the strip being curved with a radius of curvature in the range from about 100 μm to about 10 cm.
 58. A device according to claim 57, wherein the width of the strip is in the range from about 0.7 μm to about 8 μm, the thickness of the strip is in the range from about 15 nm to about 25 nm, and the radius of curvature is in the range from about 1 mm to about 10 cm, the device supporting propagation of a plasmon-polariton wave having a wavelength in the range from about 1.3 μm to about 1.7 μm.
 59. A device according to claim 58, wherein the width is about 6 μm, the thickness is about 20 nm, and the radius of curvature is about 2 cm, the device supporting propagation of a plasmon-polariton wave having a wavelength near 1.55 μm.
 60. A device according to claim 59, wherein the surrounding material comprises lithium niobate, PLZT, gallium arsenide, indium phosphide, silicon or other material having a refractive index in the range from about 2 to about 3.5, the width of the strip being in the range from about 0.15 μm to about 6 μm and the thickness of the strip being in the range from about 5 nm to about 80 nm, the strip being curved with a radius of curvature in the range from about 100 μm to about 10 cm.
 61. A device according to claim 60, wherein the width of the strip is in the range from about 0.4 μm to about 2 μm, the thickness is in the range from about 15 nm to about 40 nm, and the radius of curvature is in the range from about 1 mm to about 10 cm, the device supporting propagation of a plasmon-polariton wave having a wavelength in the range from about 1.3 μm to about 1.7 μm.
 62. A device according to claim 61, wherein the width of the strip is about 1.5 μm, the thickness of the strip is about 20 nm, and the radius of curvature of the strip is about 4 cm, the device supporting propagation of a plasmon-polariton wave having a wavelength near 1.55 μm. 